- Use another external tool to convert pdf to pdf which also adds the
extracted text as another layer into the pdf
- Although not used, the external conversion routine will now check
for an existing text file that is named as the pdf file with extension
`.txt`. If present it is included in the conversion result and will be
used as the extracted text.
- text extraction for pdf files happens now on the converted file,
because it may already contain the text from the conversion step and
thus avoids running OCR twice.
- All errors during conversion are not fatal; processing continues
without a converted file.
