Convert PDF Files

Docspell Documentation

Context and Problem Statement🔗

Some PDFs contain only images (when coming from a scanner) and therefore one is not able to click into the pdf and select text for copy&paste. Also it is not searchable in a PDF viewer. These are really shortcomings that can be fixed, especially when there is already OCR build in.

For images, this works already as tesseract is used to create the PDF files. Tesseract creates the files with an additional text layer containing the OCRed text.

Considered Options🔗

  • ocrmypdf OCRmyPDF adds an OCR text layer to scanned PDF files, allowing them to be searched

ocrmypdf🔗

This is a very nice python tool, that uses tesseract to do OCR on each page and add the extracted text as a pdf text layer to the page. Additionally it creates PDF/A type pdfs, which are great for archiving. This fixes exactly the things stated above.

Integration🔗

Docspell already has this built in for images. When converting images to a PDF (which is done early in processing), the process creates a text and a PDF file. Docspell then sets the text in this step and the text extraction step skips doing its work, if there is already text available.

It would be possible to use the --sidecar option with ocrmypdf to create a text file of the extracted text with one run, too (exactly like it works for tesseract). But for "text" pdfs, ocrmypdf writes some info-message into this text file:

[OCR skipped on page 1][OCR skipped on page 2]

Docspell cannot reliably tell, wether this is extracted text or not. It would be reqiured to load the pdf and check its contents. This is a bit of bad luck, because everything would just work already. So it requires a (small) change in the text-extraction step. By default, text extraction happens on the source file. For PDFs, text extraction should now be run on the converted file, to avoid running OCR twice.

The converted pdf file is either be a text-pdf in the first place, where ocrmypdf would only convert it to a PDF/A file; or it may be a converted file containing the OCR-ed text as a pdf layer. If ocrmypdf is disabled, the converted file and the source file are the same for PDFs.

Decision Outcome🔗

Add ocrmypdf as an optional conversion from PDF to PDF. Ocrmypdf is distributed under the GPL-3 license.