I'm building an invoicing app and need an API for PDF generation

Absolutely — for an invoicing app, a PDF generation API should ideally be simple, reliable, and designed for document workflows like invoices, quotes, receipts, and statements.

What the API should support

At minimum:

• Generate PDFs from data: invoice JSON → PDF
• Template support: reusable invoice layouts
• Customization: logo, branding, colors, tax labels, currency, page size
• Line-item tables: products/services, quantities, discounts, totals
• Attachments: optionally include terms, receipt, or supporting docs
• Delivery options:
  • return PDF bytes/base64
  • return a signed download URL
  • email the PDF to a customer
• Metadata: invoice number, customer info, issue/due dates, status
• Multi-page handling: long line-item lists
• Internationalization: currency formatting, date formats, locale
• Security: authenticated access, access tokens, expiring links

Recommended API design

1) Create a PDF from invoice data

POST /v1/pdfs/invoices

{
  "template_id": "tpl_standard_invoice",
  "invoice": {
    "invoice_number": "INV-10023",
    "issue_date": "2026-10-12",
    "due_date": "2026-11-12",
    "currency": "USD",
    "customer": {
      "name": "Acme Corp",
      "email": "billing@acme.com",
      "address": "123 Market St, San Francisco, CA"
    },
    "company": {
      "name": "My SaaS Inc",
      "address": "42 Main St, New York, NY",
      "tax_id": "US-123456789"
    },
    "items": [
      {
        "description": "Pro plan subscription",
        "quantity": 1,
        "unit_price": 99.0,
        "tax_rate": 0.07
      }
    ],
    "notes": "Thank you for your business"
  }
}

Response

{
  "pdf_id": "pdf_abc123",
  "status": "ready",
  "download_url": "https://cdn.example.com/pdfs/pdf_abc123.pdf",
  "expires_at": "2026-10-12T18:00:00Z"
}

---

2) Generate and return the PDF directly

POST /v1/pdfs/render

Use this if you want the API to return the file stream directly.

Headers:

Accept: application/pdf

Response:

• 200 OK
• Content-Type: application/pdf

---

3) Template management

GET /v1/templates
POST /v1/templates
GET /v1/templates/{template_id}
PUT /v1/templates/{template_id}

Template data could include:

{
  "name": "Standard Invoice",
  "size": "A4",
  "orientation": "portrait",
  "colors": {
    "primary": "#1E40AF",
    "text": "#111827"
  },
  "logo_url": "https://example.com/logo.png",
  "footer_text": "Payment due within 30 days"
}

---

Useful features for invoicing specifically

Line item calculations

The API should calculate:

• subtotal
• tax
• discounts
• shipping
• grand total
• amount paid / outstanding balance

Rendering options

Support:

• HTML/CSS templates
• Markdown to PDF
• programmatic layout JSON
• image embedding for logos/signatures

Webhook support

For async generation:

• pdf.generated
• pdf.failed

This is useful when PDFs are created in background jobs.

Example async workflow

1. Client sends invoice JSON
2. API returns job ID
3. Worker generates PDF
4. Webhook notifies app when ready

---

Suggested minimal OpenAPI endpoints

• POST /v1/pdfs/invoices
• GET /v1/pdfs/{pdf_id}
• GET /v1/pdfs/{pdf_id}/download
• POST /v1/templates
• GET /v1/templates
• PUT /v1/templates/{template_id}

---

Implementation options

Depending on your stack:

• Node.js: Puppeteer, Playwright, pdf-lib, PDFKit
• Python: WeasyPrint, ReportLab, CairoSVG
• Java: OpenPDF, iText
• Go: UniDoc, wkhtmltopdf wrapper
• Headless browser rendering: best for pixel-perfect invoice templates

For invoicing, HTML/CSS + headless Chromium is often the easiest way to get good-looking results.

---

If you want a production-ready API

I can help you with any of these next steps:

1. Design a full OpenAPI spec
2. Create a Node.js/Express PDF generation service
3. Build a FastAPI service
4. Design invoice JSON schema
5. Add webhook + async job queue support
6. Create an HTML invoice template

If you want, I can generate a complete OpenAPI 3.0 spec for this PDF API now.