Extract PDF invoice data from Gmail into Google Sheets using OpenAI GPT-4o-mini

Description

Quick overview
This workflow polls Gmail for unread emails with PDF attachments, extracts invoice text, uses OpenAI to pull key invoice fields, and appends the results as a structured row in a Google Sheets “Invoices” ledger.

How it works
Polls Gmail every 5 minutes for unread emails that include a PDF attachment and downloads the attachment.
Extracts text from the attached PDF file.
Sends the extracted text to OpenAI (gpt-4o-mini) to extract vendor, invoice number, dates, amounts, and currency in a strict schema.
Appends a new row to the Google Sheets “Invoices” tab with the extracted fields plus email metadata (received date, sender, and filename).

Setup
Connect Gmail, OpenAI, and Google Sheets credentials in n8n.
Create or choose a Google Sheets document with an “Invoices” sheet and columns like ReceivedDate, FromEmail, Vendor, InvoiceNumber, InvoiceDate, DueDate, Net, Tax, Total, Currency, and FileName.
Replace YOUR_GOOGLE_SHEET_ID in the Google Sheets append step with your target spreadsheet ID and confirm the sheet name is set to “Invoices”.

Requirements
Gmail, Google Sheets and OpenAI accounts (credentials connected in n8n); works with born-digital PDFs that have a text layer

Customization
Narrow the Gmail search query (sender, label, subject) or add fields to the extraction schema (PO number, line items) - both are single settings

Additional info
The extraction prompt is strict by design: the model never invents values - absent fields come back empty, amounts come back as plain numbers. Parsing a typical invoice with gpt-4o-mini costs well under $0.001. Need scanned-PDF OCR, rows posted into QuickBooks/Xero, duplicate detection or an approval step? I build extended versions with async delivery - contact links in my creator profile.