diff --git a/app/(dashboard)/invoices/[id]/page.tsx b/app/(dashboard)/invoices/[id]/page.tsx
index e4daf80c..633647f7 100644
--- a/app/(dashboard)/invoices/[id]/page.tsx
+++ b/app/(dashboard)/invoices/[id]/page.tsx
@@ -954,6 +954,15 @@ export default function InvoiceDetailPage({ params }: { params: Promise<{ id: st
             <DialogTitle>Ta bort fakturautkast</DialogTitle>
             <DialogDescription>
               Är du säker på att du vill ta bort {invoice.invoice_number ? `utkast ${invoice.invoice_number}` : 'utkastet'}? Detta kan inte ångras.
+              {invoice.invoice_number ? (
+                <span className="mt-2 block text-destructive">
+                  Löpnummer {invoice.invoice_number} är redan reserverat och kommer att bli ett permanent hopp i fakturaserien.
+                </span>
+              ) : (
+                <span className="mt-2 block text-muted-foreground">
+                  Inget löpnummer har tilldelats — fakturaserien påverkas inte.
+                </span>
+              )}
             </DialogDescription>
           </DialogHeader>
           <DialogFooter>
diff --git a/app/api/invoices/[id]/mark-sent/__tests__/route.test.ts b/app/api/invoices/[id]/mark-sent/__tests__/route.test.ts
new file mode 100644
index 00000000..820d0954
--- /dev/null
+++ b/app/api/invoices/[id]/mark-sent/__tests__/route.test.ts
@@ -0,0 +1,225 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import {
+  createMockRequest,
+  parseJsonResponse,
+  createMockRouteParams,
+  createQueuedMockSupabase,
+  makeInvoice,
+  makeCustomer,
+  makeCompanySettings,
+} from '@/tests/helpers'
+
+const { supabase: mockSupabase, enqueue, reset } = createQueuedMockSupabase()
+vi.mock('@/lib/supabase/server', () => ({
+  createClient: () => Promise.resolve(mockSupabase),
+}))
+
+vi.mock('@/lib/init', () => ({
+  ensureInitialized: vi.fn(),
+}))
+
+vi.mock('@/lib/company/context', () => ({
+  requireCompanyId: vi.fn().mockResolvedValue('company-1'),
+  getActiveCompanyId: vi.fn().mockResolvedValue('company-1'),
+}))
+
+vi.mock('@/lib/auth/require-write', () => ({
+  requireWritePermission: vi.fn().mockResolvedValue({ ok: true }),
+}))
+
+const mockRenderToBuffer = vi.fn()
+vi.mock('@react-pdf/renderer', () => ({
+  renderToBuffer: (...args: unknown[]) => mockRenderToBuffer(...args),
+  Document: vi.fn(),
+  Page: vi.fn(),
+  Text: vi.fn(),
+  View: vi.fn(),
+  StyleSheet: { create: (s: unknown) => s },
+}))
+
+vi.mock('@/lib/invoices/pdf-template', () => ({
+  InvoicePDF: vi.fn().mockReturnValue('mock-pdf-element'),
+}))
+
+const mockCreateInvoiceJournalEntry = vi.fn()
+vi.mock('@/lib/bookkeeping/invoice-entries', () => ({
+  createInvoiceJournalEntry: (...args: unknown[]) =>
+    mockCreateInvoiceJournalEntry(...args),
+}))
+
+const mockUploadDocument = vi.fn()
+vi.mock('@/lib/core/documents/document-service', () => ({
+  uploadDocument: (...args: unknown[]) => mockUploadDocument(...args),
+}))
+
+import { POST } from '../route'
+
+describe('POST /api/invoices/[id]/mark-sent — PDF archival', () => {
+  const mockUser = { id: 'user-1', email: 'test@test.se' }
+  const customer = makeCustomer({ id: 'cust-1', email: 'kund@test.se' })
+  const company = makeCompanySettings({
+    accounting_method: 'accrual',
+    entity_type: 'enskild_firma',
+  })
+  const invoice = makeInvoice({
+    id: 'inv-1',
+    invoice_number: 'F-2026010',
+    status: 'draft',
+    customer,
+    items: [
+      {
+        id: 'item-1',
+        invoice_id: 'inv-1',
+        sort_order: 0,
+        description: 'Consulting',
+        quantity: 10,
+        unit: 'tim',
+        unit_price: 1000,
+        line_total: 10000,
+        vat_rate: 25,
+        vat_amount: 2500,
+        created_at: '2024-06-15T14:30:00Z',
+      },
+    ],
+  })
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    reset()
+    mockSupabase.auth.getUser.mockResolvedValue({ data: { user: mockUser } })
+    mockRenderToBuffer.mockResolvedValue(Buffer.from('fake-pdf'))
+    mockUploadDocument.mockResolvedValue({ id: 'doc-1' })
+  })
+
+  it('archives the rendered PDF as underlag linked to the journal entry', async () => {
+    enqueue({ data: invoice, error: null }) // fetch invoice
+    enqueue({ data: null, error: null }) // status update
+    enqueue({ data: company, error: null }) // settings
+    mockCreateInvoiceJournalEntry.mockResolvedValue({ id: 'je-7' })
+    enqueue({ data: null, error: null }) // update invoice with journal_entry_id
+
+    const request = createMockRequest('/api/invoices/inv-1/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-1' }))
+    const { status, body } = await parseJsonResponse<{ success: boolean; journal_entry_id: string | null }>(response)
+
+    expect(status).toBe(200)
+    expect(body.success).toBe(true)
+    expect(body.journal_entry_id).toBe('je-7')
+
+    expect(mockRenderToBuffer).toHaveBeenCalledTimes(1)
+    expect(mockUploadDocument).toHaveBeenCalledTimes(1)
+    expect(mockUploadDocument).toHaveBeenCalledWith(
+      expect.anything(),
+      'user-1',
+      'company-1',
+      expect.objectContaining({
+        name: 'faktura-F-2026010.pdf',
+        type: 'application/pdf',
+      }),
+      expect.objectContaining({
+        upload_source: 'system',
+        journal_entry_id: 'je-7',
+      })
+    )
+  })
+
+  it('archives the PDF even when journal entry creation fails (non-blocking)', async () => {
+    enqueue({ data: invoice, error: null })
+    enqueue({ data: null, error: null })
+    enqueue({ data: company, error: null })
+    mockCreateInvoiceJournalEntry.mockRejectedValue(new Error('Period locked'))
+
+    const request = createMockRequest('/api/invoices/inv-1/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-1' }))
+    const { status, body } = await parseJsonResponse<{ success: boolean; journal_entry_id: string | null }>(response)
+
+    expect(status).toBe(200)
+    expect(body.success).toBe(true)
+    expect(body.journal_entry_id).toBeNull()
+
+    expect(mockUploadDocument).toHaveBeenCalledTimes(1)
+    expect(mockUploadDocument).toHaveBeenCalledWith(
+      expect.anything(),
+      'user-1',
+      'company-1',
+      expect.objectContaining({ name: 'faktura-F-2026010.pdf' }),
+      expect.objectContaining({
+        upload_source: 'system',
+        journal_entry_id: undefined,
+      })
+    )
+  })
+
+  it('still returns 200 when PDF archival itself fails', async () => {
+    enqueue({ data: invoice, error: null })
+    enqueue({ data: null, error: null })
+    enqueue({ data: company, error: null })
+    mockCreateInvoiceJournalEntry.mockResolvedValue({ id: 'je-8' })
+    enqueue({ data: null, error: null })
+
+    mockUploadDocument.mockRejectedValue(new Error('Storage offline'))
+
+    const request = createMockRequest('/api/invoices/inv-1/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-1' }))
+    const { status, body } = await parseJsonResponse<{ success: boolean }>(response)
+
+    expect(status).toBe(200)
+    expect(body.success).toBe(true)
+  })
+
+  it('skips PDF archival for proforma invoices', async () => {
+    const proforma = makeInvoice({
+      id: 'inv-2',
+      invoice_number: 'PF-2026005',
+      status: 'draft',
+      document_type: 'proforma',
+      customer,
+      items: invoice.items,
+    })
+
+    enqueue({ data: proforma, error: null })
+    enqueue({ data: null, error: null })
+    enqueue({ data: company, error: null })
+
+    const request = createMockRequest('/api/invoices/inv-2/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-2' }))
+    const { status, body } = await parseJsonResponse<{ success: boolean }>(response)
+
+    expect(status).toBe(200)
+    expect(body.success).toBe(true)
+    expect(mockUploadDocument).not.toHaveBeenCalled()
+    expect(mockRenderToBuffer).not.toHaveBeenCalled()
+  })
+
+  it('uses kreditfaktura filename when archiving a credit note', async () => {
+    const creditNote = makeInvoice({
+      id: 'inv-3',
+      invoice_number: 'F-2026011',
+      status: 'draft',
+      credited_invoice_id: 'inv-1',
+      customer,
+      items: invoice.items,
+    })
+
+    enqueue({ data: creditNote, error: null })
+    enqueue({ data: null, error: null })
+    enqueue({ data: company, error: null })
+    mockCreateInvoiceJournalEntry.mockResolvedValue({ id: 'je-9' })
+    enqueue({ data: null, error: null })
+    // Lookup of the original invoice's number for the credit note PDF
+    enqueue({ data: { invoice_number: 'F-2026010' }, error: null })
+
+    const request = createMockRequest('/api/invoices/inv-3/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-3' }))
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect(mockUploadDocument).toHaveBeenCalledWith(
+      expect.anything(),
+      'user-1',
+      'company-1',
+      expect.objectContaining({ name: 'kreditfaktura-F-2026011.pdf' }),
+      expect.anything()
+    )
+  })
+})
diff --git a/app/api/invoices/[id]/mark-sent/route.ts b/app/api/invoices/[id]/mark-sent/route.ts
index d8406893..8ceb8045 100644
--- a/app/api/invoices/[id]/mark-sent/route.ts
+++ b/app/api/invoices/[id]/mark-sent/route.ts
@@ -1,11 +1,14 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
+import { renderToBuffer } from '@react-pdf/renderer'
 import { createInvoiceJournalEntry } from '@/lib/bookkeeping/invoice-entries'
 import { ensureInvoiceNumber } from '@/lib/invoices/ensure-invoice-number'
 import { ensureInitialized } from '@/lib/init'
+import { InvoicePDF } from '@/lib/invoices/pdf-template'
+import { uploadDocument } from '@/lib/core/documents/document-service'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
-import type { EntityType, Invoice } from '@/types'
+import type { CompanySettings, Customer, EntityType, Invoice, InvoiceItem } from '@/types'
 
 ensureInitialized()
 
@@ -75,10 +78,10 @@ export async function POST(
     return NextResponse.json({ error: 'Kunde inte uppdatera status' }, { status: 500 })
   }
 
-  // Fetch accounting method
+  // Fetch full company settings for PDF rendering and accounting method
   const { data: settings } = await supabase
     .from('company_settings')
-    .select('accounting_method, entity_type')
+    .select('*')
     .eq('company_id', companyId)
     .single()
 
@@ -109,6 +112,53 @@ export async function POST(
     }
   }
 
+  // Render and archive the PDF as underlag so it remains retrievable even if
+  // the invoice row is later cancelled. Mirrors the send route.
+  if (isRealInvoice && settings) {
+    try {
+      const items = (invoice.items as InvoiceItem[] | null ?? []).slice().sort(
+        (a, b) => a.sort_order - b.sort_order
+      )
+
+      let originalInvoiceNumber: string | undefined
+      if (invoice.credited_invoice_id) {
+        const { data: originalInvoice } = await supabase
+          .from('invoices')
+          .select('invoice_number')
+          .eq('id', invoice.credited_invoice_id)
+          .eq('company_id', companyId)
+          .single()
+        originalInvoiceNumber = originalInvoice?.invoice_number ?? undefined
+      }
+
+      const pdfBuffer = await renderToBuffer(
+        InvoicePDF({
+          invoice: invoice as Invoice,
+          customer: invoice.customer as Customer,
+          items,
+          company: settings as CompanySettings,
+          originalInvoiceNumber,
+        })
+      )
+
+      const filename = invoice.credited_invoice_id
+        ? `kreditfaktura-${invoice.invoice_number}.pdf`
+        : `faktura-${invoice.invoice_number}.pdf`
+
+      const pdfArrayBuffer = new Uint8Array(pdfBuffer).buffer as ArrayBuffer
+      await uploadDocument(supabase, user.id, companyId, {
+        name: filename,
+        buffer: pdfArrayBuffer,
+        type: 'application/pdf',
+      }, {
+        upload_source: 'system',
+        journal_entry_id: journalEntryId ?? undefined,
+      })
+    } catch (err) {
+      console.error('Failed to archive invoice PDF on mark-sent:', err)
+    }
+  }
+
   return NextResponse.json({
     success: true,
     status: 'sent',
diff --git a/app/api/invoices/[id]/send/route.ts b/app/api/invoices/[id]/send/route.ts
index 3fc2c340..9178f350 100644
--- a/app/api/invoices/[id]/send/route.ts
+++ b/app/api/invoices/[id]/send/route.ts
@@ -110,6 +110,7 @@ export async function POST(
       .from('invoices')
       .select('invoice_number')
       .eq('id', invoice.credited_invoice_id)
+      .eq('company_id', companyId)
       .single()
 
     if (originalInvoice) {