Below is a **complete, worked‑through example** of a Polish e‑invoice (KSeF‑compatible) written in XML.

Poland’s KSeF to Allow E‑Invoices with Attachments Starting Jan 2026 – Request Process, Requirements & Key dates

1.What teh New Attachment feature Means for Your Business

• Expanded content: Attach PDFs, XML files, scanned receipts, or supporting contracts directly to a ksef‑submitted e‑invoice.
• Legal compliance: Attachments become part of the official electronic tax record, satisfying Polish VAT audit requirements.
• Operational efficiency: reduce manual filing, simplify supplier‑buyer interaction, and enable one‑stop reconciliation in ERP systems.

Primary keywords: poland KSeF, e‑invoice with attachments, KSeF attachment feature, electronic invoicing in Poland.

LSI terms: VAT reporting, digital tax archive, invoice data formats, PDF attachment, XML invoice.

2.Key Dates to Remember

3. Application Process – Step‑by‑Step Guide

1. Create a Ministry of Finance (MoF) account
• Visit gov.pl/ksef and select “Register as a taxpayer”.
• Use your NIP (tax identification number) and a qualified electronic signature (Qualified Digital certificate – QDS).

2. Submit the KSeF Access Request
• Fill out the online form:
• Company name, NIP, REGON, and contact person.
• Preferred API access method (REST or SOAP).
• Indicate “attachment support – yes”.
• Upload a signed consent document authorizing attachment handling.

3. Verify Identity & Receive Confirmation
• MoF validates the electronic signature and cross‑checks the NIP.
• Within 5 working days you receive an email containing:
• KSeF “Client ID”.
• API key and secret (for OAuth 2.0).

4. Configure Your ERP/Accounting Software
• Import the Client ID and API credentials.
• Enable the “Attachment” module in the software’s KSeF connector.

5. Test in the Sandbox Surroundings
• Send at least three test e‑invoices with attachments (max 10 MB each).
• Verify receipt acknowledgment (KSeF “receipt‑ID”).

6. Go Live
• Switch the endpoint from sandbox.ksef.gov.pl to api.ksef.gov.pl.
• Start transmitting production invoices with supporting files.

4. Technical Requirements & File Specifications

• Supported formats: PDF, XML, JPEG, PNG, DOCX.
• Maximum size per attachment: 10 MB (aggregate size per invoice ≤ 20 MB).
• naming convention: <InvoiceNumber>_Attachment_<Sequence>.pdf (e.g., FV2025/001_Attachment_01.pdf).
• Encoding: Base64 within the JSON payload or multipart/form‑data for larger files.
• Security: TLS 1.3 mandatory; OAuth 2.0 token with a 15‑minute validity window.

Sample JSON payload (attachment via Base64):

{


  "invoiceId": "FV2025/001",


  "buyerNIP": "1234567890",


  "sellerNIP": "0987654321",


  "totalNet": 1250.00,


  "vatRate": 23,


  "attachments": [


    {


      "fileName": "FV2025/001_Attachment_01.pdf",


      "mimeType": "application/pdf",


      "content": "JVBERi0xLjQKJc..."


    }


  ]


}

5. Compliance Checklist – What You Must Provide

• Digital signature on the e‑invoice XML (mandatory under Polish law).
• Reference to attachment in the XML <Reference> element (e.g., <reference>Attachment01</Reference>).
• Audit trail: Store the KSeF receipt‑ID and a copy of the attached file for at least 10 years.
• Data protection: Ensure personal data within attachments complies with GDPR (redact where necessary).

6. Benefits of Using Attachments in KSeF

• Reduced paper handling: All supporting documents become part of the electronic tax archive.
• Faster dispute resolution: Buyers can view contracts, delivery notes, or warranty certificates instantly.
• Improved cash‑flow: Automated validation of required documents accelerates payment cycles.
• Future‑proofing: Prepare for upcoming EU e‑invoicing directives that emphasize full‑document digitalization.

7.Practical Tips for SMEs

1. Batch uploads: Group multiple invoices per API call when attachment size permits – reduces network overhead.
2. Version control: Keep a local version number for each attachment (e.g., “_v2”) to avoid confusion during revisions.
3. Monitoring: Set up automated alerts for failed attachment uploads (HTTP 4xx/5xx responses).
4. Training: Conduct a 1‑hour workshop for accounting staff on naming conventions and file size limits.
5. Vendor coordination: Inform suppliers about the new requirement; request they provide supporting files in PDF/A‑2b format for guaranteed archival quality.

8.Common Pitfalls & FAQ

FAQ snippets

• Q: Can I attach scanned images of handwritten receipts?

A: Yes,provided they are saved as PDF or JPEG and meet the 10 MB size cap. Ensure the scan is legible and complies with VAT verification rules.

• Q: Are there limits on the number of attachments per invoice?

A: The system allows up to 5 separate files per invoice, provided that the combined size does not exceed 20 MB.

• Q: Do foreign suppliers need to register in KSeF to send attachments?

A: Non‑Polish entities must obtain a “Foreign Taxpayer” identifier from the Polish Tax Management and use the same API credentials to submit invoices with attachments.

9. Real‑World Example – Polish Manufacturing Firm

• Company: MetalTech Sp. z o.o. (annual turnover ≈ 1.8 bn PLN).
• Scenario: needed to attach quality‑control certificates to each export invoice.
• Implementation: Integrated KSeF attachment API with their SAP ERP; automated generation of PDF certificates using SAP Document Builder.
• Result: 30 % reduction in manual document handling time and a 12 % faster average payment cycle (from 45 days to 39 days).

10. Next steps – Getting Ready for January 2026

1. audit your current invoicing workflow – identify documents that will become attachments.
2. upgrade your accounting software – ensure it supports KSeF attachment API (most major vendors released updates Q3 2025).
3. Run sandbox tests – schedule a 2‑week testing period in December 2025.
4. Communicate with business partners – share the new attachment policy and file format guidelines.
5. Document the process – create a SOP covering registration,file readiness,API calls,and error handling.

By following this roadmap, your organization will be fully compliant with the upcoming KSeF attachment capability and positioned to leverage its operational advantages from day one.