Automated PDF Invoice Processing & Approval Flow (n8n + OpenAI + Google Sheets)

PDF invoice → text extraction → GPT structured parsing → email approval → Google Sheets logging

Who

Finance teams and ops managers at SMBs who need automated invoice intake and approval audit trails

You get

Receive PDF invoices via Drive, Gmail, or form; AI extracts fields and categories; email form approval; auto-write to Google Sheets

You need

n8n · google-drive · gmail · google-sheets

Steps

1. Configure input channels and credentials

   Set up one or all three triggers in n8n: 1) Google Drive Trigger: specify folder ID to watch (fileCreated, poll every minute); 2) Gmail Trigger: enable downloadAttachments with prefix attachment_; 3) Form Trigger: add a file field restricted to .pdf. Connect Google Drive, Gmail, Google Sheets OAuth2 credentials and OpenAI API Key.

2. Configure the AI parser node

   Add an AI Agent node (Invoice Parser AI Agent), connect OpenAI Chat Model (default gpt-4o-mini) and Structured Output Parser. Paste the system prompt into the agent’s text field to extract invoice fields and categorize.

   Execution prompt · Source text。Invoice Parser AI Agent node text parameter (promptType=define)

   Copy prompt

   =You are an intelligent invoice parser. I will give you raw text extracted from a PDF invoice. 
   
   ========================
   START OF RAW INVOICE TEXT
   
   {{ $json.text }}
   
   END OF RAW INVOICE TEXT
   ========================
   
   Your job is to:
   
   1. Extract key information from the invoice such as:
      - invoice_number
      - invoice_date
      - due_date (if available)
      - vendor_name
      - total_amount
      - currency (e.g., USD, IDR, etc.)
      - items (as a list of item descriptions and their amounts)
      - tax (if available)
   
   2. Detect the invoice **category**, such as:
      - Utilities
      - Office Supplies
      - Travel
      - Software
      - Food & Beverage
      - Others (if unknown)
   
   3. Return the result in this exact JSON format:
   
   ```json
   {
     "invoice_number": "",
     "invoice_date": "",
     "due_date": "",
     "vendor_name": "",
     "total_amount": "",
     "currency": "",
     "items": [
       {
         "description": "",
         "amount": ""
       }
     ],
     "tax": "",
     "category": ""
   }

3. Set up the approval email

   Add a Gmail node (Send Invoice for Approval), set operation to sendAndWait and responseType to customForm. Configure recipient address, subject (with vendor name variable), and form fields: dropdown Approve Invoice? (Yes/No), required text Reviewed By, textarea Approval Notes.

4. Configure approval branching

   Add an If node (Check Approval Decision), condition: $json.data['Approve Invoice?'] equals Yes. True branch → Google Sheets append; False branch → rejection alert email.

5. Configure logging and rejection alert

   Google Sheets node (Insert Invoice Data): operation=append, specify document ID and sheet name, map fields Invoice Number, Date, Due Date, Vendor, Total, Currency, Items, Tax, Category, Approved, Approval Notes, Reviewed By from AI Agent output and approval form data. Rejection branch: Gmail node (Send Rejection Alert) sends plain-text email to finance team with vendor name, reviewer, and notes.

You are an n8n workflow setup assistant. The user wants to replicate the Automated PDF Invoice Processing & Approval Flow. Assist in this order: 1) Confirm they have an n8n instance, Google OAuth credentials, and an OpenAI API Key; 2) Guide them to import the workflow JSON or build manually; 3) Emphasize copying the Google Sheet template and updating the document ID; 4) Remind them to replace [email protected] with real recipients; 5) Suggest testing end-to-end with a sample PDF first. Do not execute any API calls or send real emails on their behalf.