Document Processing

Document processing

File in, content out — or data in, file out. You open a byte stream (PDF, DOCX, scan) and either pull the content out, or you build a new document from a template and a data dict. That is the whole job: the deliverable is bytes of a document or the literal content of one.

The boundary test, apply it first:

• Deliverable is raw text / Markdown / table cells / a generated file → you are in the right place.
• Deliverable is a typed object matching a schema ({parties: [...], total: 1234.50}) → that is structured-extraction. This skill stops at "clean Markdown out of the file"; the schema-constrained extraction runs on that Markdown.

Everything else routes too: signing with an audit trail → e-signature, spreadsheet grids/formulas/XLSX-as-data → spreadsheet-ops, indexing for cross-document Q&A → rag (this skill produces the text rag ingests, it does not index it), downloading the files off a site → data-scraper.

Step 0 — does the PDF have a text layer?

The most expensive mistake in this skill is OCR'ing a PDF that already has a text layer. A digital PDF (exported from Word, a browser, a report tool) carries selectable text — extracting it is free, instant, and lossless. OCR is slow, costs money or GPU, and introduces errors. Never OCR a PDF you can extract.

Check before you pick an engine:

import pdfplumber

with pdfplumber.open("doc.pdf") as pdf:
    txt = pdf.pages[0].extract_text() or ""

if len(txt.strip()) > 20:
    print("text layer present -> extract directly (pdfplumber / pypdf)")
else:
    print("image-only or empty -> this is an OCR job")

If extract_text() returns empty (or near-empty) across the first few pages, it is a scan or image-only PDF and you go to the OCR branch. Symptom from the user's side: "the text copies out as garbage / random symbols" usually means a broken/embedded font, not a missing text layer — try pypdf extraction too before assuming OCR.

Engine selection

Extraction recipes

Text + tables with pdfplumber, straight to CSV:

import csv
import pdfplumber

rows = []
with pdfplumber.open("invoice.pdf") as pdf:
    for page in pdf.pages:
        for table in page.extract_tables():
            rows.extend(table)

with open("out.csv", "w", newline="") as f:
    csv.writer(f).writerows(rows)

Raw text, merge, split, rotate with pypdf:

from pypdf import PdfReader, PdfWriter

# raw text
text = "\n".join(p.extract_text() or "" for p in PdfReader("doc.pdf").pages)

# merge two files
w = PdfWriter()
for src in ("a.pdf", "b.pdf"):
    w.append(src)
with open("merged.pdf", "wb") as f:
    w.write(f)

# split first 3 pages + rotate one
w2 = PdfWriter()
reader = PdfReader("doc.pdf")
for page in reader.pages[:3]:
    w2.add_page(page)
w2.pages[0].rotate(90)
with open("first3.pdf", "wb") as f:
    w2.write(f)

Form filling (AcroForm)

Dump the field names first — guessing them is the #1 reason a fill silently does nothing:

from pypdf import PdfReader

fields = PdfReader("form.pdf").get_fields() or {}
for name, f in fields.items():
    print(name, "->", f.get("/FT"))  # /Tx text, /Btn checkbox/radio, /Ch choice

Then write the values. Set auto_regenerate=False and bake with flatten=True if it must not be editable:

from pypdf import PdfReader, PdfWriter

reader = PdfReader("form.pdf")
writer = PdfWriter()
writer.append(reader)

for page in writer.pages:
    writer.update_page_form_field_values(
        page,
        {"applicant_name": "Eric Risco", "agree": "/Yes"},  # checkbox = its on-state
        auto_regenerate=False,  # else a spurious "save changes?" prompt fires on open
    )

# flatten=True bakes the values and drops the editable widgets
with open("filled.pdf", "wb") as f:
    writer.write(f)

auto_regenerate defaults to True for legacy reasons, and you almost never want it. Checkbox/radio values are the field's /V on-state (often /Yes), not True — read the field to find it.

Generation

DOCX from a Word template you authored and tagged with Jinja2 ({{ client }}, {% tr for row in items %} on a table row, InlineImage for pictures):

from docxtpl import DocxTemplate

doc = DocxTemplate("contract_template.docx")
doc.render({
    "client": "Acme SL",
    "date": "2026-06-02",
    "items": [{"desc": "Audit", "amount": "1.200,00 €"}],
})
doc.save("contract_2026-06-02.docx")

PDF from scratch with ReportLab Platypus:

from reportlab.lib.pagesizes import A4
from reportlab.platypus import SimpleDocTemplate, Paragraph, Spacer
from reportlab.lib.styles import getSampleStyleSheet

styles = getSampleStyleSheet()
doc = SimpleDocTemplate("report.pdf", pagesize=A4)
doc.build([
    Paragraph("Quarterly Report", styles["Title"]),
    Spacer(1, 12),
    Paragraph("Generated automatically from the data dict.", styles["BodyText"]),
])

OCR

Branch on cost and privacy. Local, no API budget, or data must not leave the machine → Docling/Marker. Messy scans, handwriting, brutal tables, and an API budget is fine → Mistral OCR.

Local with Docling (wraps Tesseract / RapidOCR, exports Markdown preserving tables):

from docling.document_converter import DocumentConverter

result = DocumentConverter().convert("scan.pdf")
markdown = result.document.export_to_markdown()
open("scan.md", "w").write(markdown)

Hosted with Mistral OCR (~$2 / 1,000 pages, 50% off via Batch API; outputs interleaved text+images as Markdown):

from mistralai import Mistral

client = Mistral(api_key=os.environ["MISTRAL_API_KEY"])
resp = client.ocr.process(
    model="mistral-ocr-2512",
    document={"type": "document_url", "document_url": signed_url},
)
markdown = "\n\n".join(p.markdown for p in resp.pages)

Never trust OCR output blind. OCR confuses 0/O, 1/l/I, and drops or shifts decimal points — a 1.234,50 can come back as 1234,50 or 1,234.50. Always spot-check totals, dates, and ID numbers against the rendered page before you hand the text downstream. For clean scans with no budget, plain pytesseract is the zero-cost baseline, but it is weak on layout/tables versus the pipelines above.

Scale

Batch jobs: parallelize per-file, cap concurrency on the hosted API (rate limits + cost), and use Mistral's Batch API for the 50% discount on large runs. Engine install matrix, exact version pins, the full licensing table, the Docling-vs-Marker-vs-Mistral feature/cost comparison, and troubleshooting (encrypted PDFs, mangled AcroForm field names, multi-column reading order, CJK/handwriting) live in references/engines.md — read it before a non-trivial install.

Anti-patterns