Fill PDFs from JSON in n8n: JustFill + Gemini Vision Workflow

Fill PDFs from JSON in n8n—with JustFill field detection and Google Gemini Vision review

This workflow takes an uploaded PDF and a Data JSON payload in an n8n form, automatically maps your JSON values to the correct PDF fields using JustFill, and then uses Google Gemini Vision to visually verify the filled result before generating a downloadable PDF.

What this workflow does

• Collects inputs via an n8n form: users upload a PDF and submit a “Data JSON” object.
• Requests a JustFill upload slot: the workflow uploads the PDF to JustFill, opens it, and detects fillable fields to create a workspace.
• Generates labelled previews: it renders labelled page previews for all pages containing detected fields.
• Maps JSON keys to JustFill field IDs: the workflow sends the labelled previews plus the user’s JSON keys to Google Gemini Vision, which maps each data key to the appropriate field ID.
• Visually validates placement and readability: it renders filled previews with the proposed values and asks Gemini to review across pages.
• Fills and returns a download link: if Gemini approves with no issues, the workflow fills the PDF in JustFill and returns a redirect link to download the completed document.

Use cases

• Contract or application automation: fill multi-page PDFs from structured JSON data while ensuring the output is readable.
• Operations and SaaS workflows: reduce manual data entry for forms by validating field placement before final PDF generation.
• Workflow prototyping for automation engineers: create a reliable “PDF + JSON → approved filled PDF” pipeline with visual review.

Technical details

• Integrations: JustFill (request_file_upload, open_pdf, render_preview, render_filled_preview, fill_pdf) and Google Gemini Vision (field mapping + visual approval).
• n8n nodes / building blocks: Set, Code, Form, Sticky Note, Form Trigger, HTTP Request.
• Setup requirements: add a JustFill API key and Google Gemini API key in the Config step (optionally adjust the model name), confirm your JustFill MCP tools are accessible, then customize the n8n form fields and sample JSON.