Pdf form filler

PDF Form Filler

Fill PDF forms using pypdf (available in the environment).

Workflow

1. Extract field names: Read the PDF and list all form fields with their full hierarchical paths
2. Identify field types: /Tx = text field, /Btn = checkbox/button
3. Map data to fields: Match user-provided data to the correct field paths
4. Fill and save: Use pypdf.PdfWriter to write values

Key Code Pattern

import pypdf

reader = pypdf.PdfReader("input.pdf")
writer = pypdf.PdfWriter()
writer.append(reader)

# Fill text fields - use the SHORT field name (leaf), not full path
# writer.update_page_form_field_values(writer.pages[page_idx], {"FieldName[0]": "value"})

# For checkboxes, update the annotation directly:
# Find the annotation on the page, then set /V and /AS to the checked state
for page in writer.pages:
    for annot in page.get("/Annots", []):
        obj = annot.get_object()
        if obj.get("/T") == "CheckboxName[0]":
            obj.update({
                pypdf.generic.NameObject("/V"): pypdf.generic.NameObject("/1"),
                pypdf.generic.NameObject("/AS"): pypdf.generic.NameObject("/1"),
            })

writer.write("output.pdf")

Checkbox States

• Check the /AP -> /N dictionary for valid appearance states (e.g., /1, /2, /Off)
• Set both /V and /AS to the desired state
• /Off typically means unchecked

Tips

• Field names in annotations use the SHORT leaf name (e.g., PlaintiffName1[0]), not the full hierarchical path
• update_page_form_field_values works for text fields on a specific page
• For checkboxes, iterate annotations and update directly
• Some PDFs have duplicate field names across pages (e.g., Plaintiff[0] on pages 2, 3, 4) - handle each page separately