Fill Existing PDF Forms from n8n Without Rebuilding the Template

Description and use

The updated n8n workflow uses strict normalization of PDF field names to ensure accurate form filling without rebuilding templates.

Problem

Earlier n8n workflows matched PDF field names loosely, causing data to populate incorrect boxes when exact names were missing.

Outcome

The updated workflow correctly maps JSON keys to PDF fields, rejects unknown keys, and produces clean PDFs with data in the intended boxes.

Workflow

• Import workflow JSON from GitHub
• Create JustFill API key and store in Config node
• Set up Form Trigger to receive PDF + JSON
• Request one‑time upload URL via HTTP Request node
• POST PDF binary to JustFill
• Open PDF via HTTP Request
• Map field names using Code node with strict normalization
• Fill PDF via HTTP Request
• Redirect to download URL

Evidence

The September 5, 2026 check ran the workflow's actual mapping code against production MCP upload, open, and fill responses. All three values appeared in the expected boxes in a clean PDF, which was also checked visually.

Source

The old mapping node used find() with a substring fallback. It also stripped everything outside a-z and 0-9 when comparing names, which lost non‑ASCII letters.

Source