diff --git a/app/api/documents/[id]/inline/__tests__/route.test.ts b/app/api/documents/[id]/inline/__tests__/route.test.ts
new file mode 100644
index 00000000..5be74ca2
--- /dev/null
+++ b/app/api/documents/[id]/inline/__tests__/route.test.ts
@@ -0,0 +1,108 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import {
+  parseJsonResponse,
+  createMockRouteParams,
+  createQueuedMockSupabase,
+} from '@/tests/helpers'
+
+const { supabase: mockSupabase, enqueue, reset } = createQueuedMockSupabase()
+
+const requireAuthMock = vi.fn()
+vi.mock('@/lib/auth/require-auth', () => ({
+  requireAuth: (...args: unknown[]) => requireAuthMock(...args),
+}))
+
+const downloadMock = vi.fn()
+vi.mock('@/lib/supabase/server', () => ({
+  createServiceClient: () => ({
+    storage: {
+      from: () => ({ download: downloadMock }),
+    },
+  }),
+}))
+
+import { GET } from '../route'
+import { NextResponse } from 'next/server'
+
+const mockUser = { id: 'user-1', email: 'test@test.se' }
+
+// NFD filename as macOS/iOS uploads produce them: o + combining diaeresis
+// U+0308 (char code 776). This is the exact shape that made the raw header
+// build throw in prod (undici Headers require code units <= 0xFF).
+const NFD_FILE_NAME = 'kvitto fo\u0308rvaring.pdf'
+
+function makeDoc(overrides: Record<string, unknown> = {}) {
+  return {
+    id: 'doc-1',
+    company_id: 'company-1',
+    file_name: NFD_FILE_NAME,
+    mime_type: 'application/pdf',
+    storage_path: 'documents/user-1/doc-1.pdf',
+    ...overrides,
+  }
+}
+
+function makeReq() {
+  return new Request('http://localhost/api/documents/doc-1/inline')
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  reset()
+  requireAuthMock.mockResolvedValue({ user: mockUser, supabase: mockSupabase, error: null })
+  downloadMock.mockResolvedValue({ data: new Blob(['%PDF-1.4']), error: null })
+})
+
+describe('GET /api/documents/[id]/inline', () => {
+  it('returns 401 when not authenticated', async () => {
+    requireAuthMock.mockResolvedValue({
+      user: null,
+      supabase: mockSupabase,
+      error: NextResponse.json({ error: 'Unauthorized' }, { status: 401 }),
+    })
+    const res = await GET(makeReq(), createMockRouteParams({ id: 'doc-1' }))
+    const { status } = await parseJsonResponse(res)
+    expect(status).toBe(401)
+  })
+
+  it('returns 404 when the document is not found', async () => {
+    enqueue({ data: null, error: { message: 'not found' } }) // doc lookup
+    const res = await GET(makeReq(), createMockRouteParams({ id: 'doc-1' }))
+    const { status, body } = await parseJsonResponse<{ error: string }>(res)
+    expect(status).toBe(404)
+    expect(body.error).toBe('Document not found')
+  })
+
+  it('returns 404 when the user is not a member of the document company', async () => {
+    enqueue({ data: makeDoc(), error: null }) // doc lookup
+    enqueue({ data: null, error: null }) // membership lookup
+    const res = await GET(makeReq(), createMockRouteParams({ id: 'doc-1' }))
+    const { status } = await parseJsonResponse(res)
+    expect(status).toBe(404)
+  })
+
+  it('returns 500 when the storage download fails', async () => {
+    enqueue({ data: makeDoc(), error: null })
+    enqueue({ data: { company_id: 'company-1' }, error: null })
+    downloadMock.mockResolvedValue({ data: null, error: { message: 'boom' } })
+    const res = await GET(makeReq(), createMockRouteParams({ id: 'doc-1' }))
+    const { status } = await parseJsonResponse(res)
+    expect(status).toBe(500)
+  })
+
+  it('streams the file with an RFC 5987 Content-Disposition for an NFD filename', async () => {
+    enqueue({ data: makeDoc(), error: null })
+    enqueue({ data: { company_id: 'company-1' }, error: null })
+
+    const res = await GET(makeReq(), createMockRouteParams({ id: 'doc-1' }))
+
+    expect(res.status).toBe(200)
+    const disposition = res.headers.get('Content-Disposition') ?? ''
+    expect(disposition).toContain('inline')
+    // Extended form carries the NFC-composed UTF-8 percent-encoded name.
+    expect(disposition).toContain(`filename*=UTF-8''kvitto%20f%C3%B6rvaring.pdf`)
+    // ASCII fallback replaces the non-ASCII character.
+    expect(disposition).toContain('filename="kvitto f_rvaring.pdf"')
+    expect(res.headers.get('Content-Type')).toBe('application/pdf')
+  })
+})
diff --git a/app/api/documents/[id]/inline/route.ts b/app/api/documents/[id]/inline/route.ts
index cb9a166b..d90baa56 100644
--- a/app/api/documents/[id]/inline/route.ts
+++ b/app/api/documents/[id]/inline/route.ts
@@ -1,6 +1,7 @@
 import { NextResponse } from 'next/server'
 import { requireAuth } from '@/lib/auth/require-auth'
 import { createServiceClient } from '@/lib/supabase/server'
+import { contentDisposition } from '@/lib/api/content-disposition'
 
 /**
  * GET /api/documents/:id/inline
@@ -87,13 +88,14 @@ export async function GET(
     )
   }
 
-  const safeFileName = doc.file_name.replace(/[\r\n"]/g, '_')
-
   return new NextResponse(blob, {
     status: 200,
     headers: {
       'Content-Type': resolveContentType(doc.file_name, doc.mime_type),
-      'Content-Disposition': `inline; filename="${safeFileName}"`,
+      // RFC 5987 dual form: NFD filenames from macOS/iOS uploads contain
+      // combining marks (> 0xFF), which undici Headers reject as non-
+      // ByteString values; splicing the raw name here 500ed the route.
+      'Content-Disposition': contentDisposition('inline', doc.file_name),
       'Cache-Control': 'private, max-age=300',
       // Block MIME sniffing: Content-Type is derived from DB metadata
       // (with extension fallback for legacy rows), never from response
diff --git a/app/api/payslip/[token]/pdf/route.ts b/app/api/payslip/[token]/pdf/route.ts
index a8e908d9..521e13a3 100644
--- a/app/api/payslip/[token]/pdf/route.ts
+++ b/app/api/payslip/[token]/pdf/route.ts
@@ -4,6 +4,7 @@ import { createServiceClientNoCookies } from '@/lib/auth/api-keys'
 import { resolvePayslipToken, isValidPayslipTokenFormat } from '@/lib/salary/payslips/links'
 import { buildPayslipData, payslipFileName } from '@/lib/salary/payslips/build-payslip-data'
 import { PayslipPDF } from '@/lib/salary/pdf/payslip-template'
+import { contentDisposition } from '@/lib/api/content-disposition'
 
 // In-memory rate limiting per token (pattern from /api/calendar/feed).
 const rateLimitMap = new Map<string, { count: number; resetAt: number }>()
@@ -118,7 +119,10 @@ export async function GET(
   return new Response(buffer as unknown as BodyInit, {
     headers: {
       'Content-Type': 'application/pdf',
-      'Content-Disposition': `attachment; filename="${fileName}"`,
+      // RFC 5987 dual form: employee names with non-Latin-1 characters
+      // (e.g. NFD combining marks) would otherwise make undici reject
+      // the header value and crash the response.
+      'Content-Disposition': contentDisposition('attachment', fileName),
       'Cache-Control': 'no-store',
     },
   })
diff --git a/app/api/salary/runs/[id]/payslips/[employeeId]/pdf/route.ts b/app/api/salary/runs/[id]/payslips/[employeeId]/pdf/route.ts
index 60ff6a1d..8f035869 100644
--- a/app/api/salary/runs/[id]/payslips/[employeeId]/pdf/route.ts
+++ b/app/api/salary/runs/[id]/payslips/[employeeId]/pdf/route.ts
@@ -5,6 +5,7 @@ import { getCompanyDisplayName } from '@/lib/company/context'
 import { renderToBuffer } from '@react-pdf/renderer'
 import { PayslipPDF } from '@/lib/salary/pdf/payslip-template'
 import { buildPayslipData, payslipFileName } from '@/lib/salary/payslips/build-payslip-data'
+import { contentDisposition } from '@/lib/api/content-disposition'
 
 ensureInitialized()
 
@@ -81,7 +82,10 @@ export const GET = withRouteContext<{ params: Promise<{ id: string; employeeId:
     return new Response(buffer as unknown as BodyInit, {
       headers: {
         'Content-Type': 'application/pdf',
-        'Content-Disposition': `inline; filename="${fileName}"`,
+        // RFC 5987 dual form: employee names with non-Latin-1 characters
+        // (e.g. NFD combining marks) would otherwise make undici reject
+        // the header value and crash the response.
+        'Content-Disposition': contentDisposition('inline', fileName),
       },
     })
   },
diff --git a/lib/api/__tests__/content-disposition.test.ts b/lib/api/__tests__/content-disposition.test.ts
new file mode 100644
index 00000000..1857e69a
--- /dev/null
+++ b/lib/api/__tests__/content-disposition.test.ts
@@ -0,0 +1,91 @@
+import { describe, it, expect } from 'vitest'
+import { contentDisposition } from '../content-disposition'
+
+describe('contentDisposition', () => {
+  it('passes a plain ASCII filename through unchanged in both forms', () => {
+    expect(contentDisposition('inline', 'kvitto.pdf')).toBe(
+      `inline; filename="kvitto.pdf"; filename*=UTF-8''kvitto.pdf`,
+    )
+  })
+
+  it('supports the attachment type', () => {
+    expect(contentDisposition('attachment', 'report.pdf')).toBe(
+      `attachment; filename="report.pdf"; filename*=UTF-8''report.pdf`,
+    )
+  })
+
+  it('produces a ByteString-safe value for an NFD Swedish filename', () => {
+    // macOS/iOS NFD upload: base letter + combining diaeresis U+0308 (776),
+    // the exact shape that crashed the inline route in prod.
+    const nfd = 'kvitto fo\u0308rvaring.pdf'
+    expect(nfd.charCodeAt(9)).toBe(776)
+
+    const header = contentDisposition('inline', nfd)
+
+    const maxCode = Math.max(...[...header].map((c) => c.charCodeAt(0)))
+    expect(maxCode).toBeLessThanOrEqual(255)
+    // The whole header is in fact printable ASCII.
+    expect(/^[\x20-\x7e]*$/.test(header)).toBe(true)
+    // filename* carries the NFC-composed UTF-8 percent-encoding of the name.
+    expect(header).toContain(`filename*=UTF-8''kvitto%20f%C3%B6rvaring.pdf`)
+    // The quoted fallback replaces the non-ASCII character with _.
+    expect(header).toContain('filename="kvitto f_rvaring.pdf"')
+  })
+
+  it('is stable across NFD and NFC inputs of the same name', () => {
+    const nfd = 'lo\u0308n.pdf' // o + combining diaeresis
+    const nfc = 'l\u00f6n.pdf' // precomposed \u00f6
+    expect(contentDisposition('inline', nfd)).toBe(contentDisposition('inline', nfc))
+  })
+
+  it('does not throw when used as an undici Headers value', () => {
+    const nfd = 'lo\u0308nespec_a\u030agren.pdf'
+    expect(
+      () => new Headers({ 'Content-Disposition': contentDisposition('inline', nfd) }),
+    ).not.toThrow()
+  })
+
+  it('neutralizes quote and CRLF header injection', () => {
+    const header = contentDisposition('attachment', 'evil"\r\nSet-Cookie: x=y.pdf')
+    expect(header).not.toContain('\r')
+    expect(header).not.toContain('\n')
+    expect(header).toContain('filename="evil___Set-Cookie: x=y.pdf"')
+    // The extended form percent-encodes them instead of emitting them raw.
+    expect(header).toContain('%0D%0A')
+  })
+
+  it('replaces backslash in the quoted fallback', () => {
+    expect(contentDisposition('inline', 'a\\b.pdf')).toContain('filename="a_b.pdf"')
+  })
+
+  it(`percent-escapes ! ' ( ) * which encodeURIComponent leaves bare`, () => {
+    const header = contentDisposition('inline', "a!'()*.pdf")
+    expect(header).toContain(`filename*=UTF-8''a%21%27%28%29%2A.pdf`)
+  })
+
+  it('sanitizes a lone high surrogate instead of throwing', () => {
+    let header = ''
+    expect(() => {
+      header = contentDisposition('attachment', '\uD800')
+    }).not.toThrow()
+    // The lone surrogate becomes U+FFFD: _ in the fallback, percent-encoded
+    // UTF-8 (%EF%BF%BD) in the extended form.
+    expect(header).toBe(`attachment; filename="_"; filename*=UTF-8''%EF%BF%BD`)
+    expect(() => new Headers({ 'Content-Disposition': header })).not.toThrow()
+  })
+
+  it('sanitizes an embedded unpaired surrogate and keeps the rest of the name', () => {
+    let header = ''
+    expect(() => {
+      header = contentDisposition('inline', 'a\uD800b')
+    }).not.toThrow()
+    expect(header).toBe(`inline; filename="a_b"; filename*=UTF-8''a%EF%BF%BDb`)
+    expect(() => new Headers({ 'Content-Disposition': header })).not.toThrow()
+  })
+
+  it('keeps a valid surrogate pair (emoji) working unchanged', () => {
+    const header = contentDisposition('inline', 'r😀.pdf')
+    expect(header).toBe(`inline; filename="r__.pdf"; filename*=UTF-8''r%F0%9F%98%80.pdf`)
+    expect(() => new Headers({ 'Content-Disposition': header })).not.toThrow()
+  })
+})
diff --git a/lib/api/content-disposition.ts b/lib/api/content-disposition.ts
new file mode 100644
index 00000000..8d17d955
--- /dev/null
+++ b/lib/api/content-disposition.ts
@@ -0,0 +1,43 @@
+/**
+ * RFC 6266 Content-Disposition builder with RFC 5987 extended filename
+ * encoding.
+ *
+ * undici (the fetch/Headers implementation in the Next.js runtime) requires
+ * header values to be ByteStrings: every code unit <= 0xFF. Splicing a raw
+ * filename into the header therefore throws for any non-Latin-1 character,
+ * e.g. the NFD combining diaeresis (U+0308) that macOS/iOS uploads put in
+ * Swedish filenames, turning the whole response into a 500.
+ *
+ * The dual form emitted here is:
+ *
+ *   <type>; filename="<ascii fallback>"; filename*=UTF-8''<percent-encoded>
+ *
+ * Legacy clients read `filename`; modern browsers prefer `filename*`
+ * (RFC 6266 section 4.3) and decode the original UTF-8 name.
+ */
+export function contentDisposition(
+  type: 'inline' | 'attachment',
+  filename: string,
+): string {
+  // Lone/unpaired UTF-16 surrogates survive normalize('NFC') and make
+  // encodeURIComponent below throw a URIError, which would turn the download
+  // response into the very 500 this helper exists to prevent. Replace them
+  // with U+FFFD first so the function always returns a valid header value.
+  // Then normalize NFD (macOS/iOS) to NFC so precomposed characters encode
+  // as themselves instead of base letter + combining mark.
+  const normalized = filename.toWellFormed().normalize('NFC')
+
+  // ASCII fallback for the quoted-string form: anything outside printable
+  // ASCII, plus the quoted-string specials " and \, becomes _. This also
+  // neutralizes CR/LF header injection.
+  const fallback = normalized.replace(/[^\x20-\x7e]|["\\]/g, '_')
+
+  // RFC 5987 value-chars: encodeURIComponent covers everything except
+  // ! ' ( ) * which it leaves bare but RFC 5987 forbids unencoded.
+  const encoded = encodeURIComponent(normalized).replace(
+    /[!'()*]/g,
+    (c) => `%${c.charCodeAt(0).toString(16).toUpperCase()}`,
+  )
+
+  return `${type}; filename="${fallback}"; filename*=UTF-8''${encoded}`
+}