I’m grateful to unoconv for an idea and great README explaining multiple problems I can come across. Apparently, LibreOffice can be run in haedless mode and supports conversion between various formats, sweet! As execution time isn't my problem I have been only concerned whether it’s possible to run LibreOffice without X display. That sound like a solid solution for my use case where I care more about quality than anything else.

Universal Office Converter is a library that’s converting any document format supported by LibreOffice/OpenOffice. That said, I know I'd fail miserably trying to achieve 1:1 conversion.

Those are good choices and a lot of fun to read or write files. Python has a few great libraries to work with DOCX ( python-dox) and PDF files ( PyPDF2, pdfrw). PDF is much easier when you care more about how a document is displayed than its abilities for further modifications. DOCX is a binary file which is, unlike XLSX, not famous for being easy to integrate into your application.

Whether you like it not, it makes sense considering that 1.2 billion people use Microsoft Office although a definition of “use” is quite vague in this case. When you ask someone to send you a contract or a report there is a high probability that you’ll get a DOCX file.

Converting docx to pdf