You can open a PDF file with Word (at least with Word 2016), which will attempt to convert the PDF file into a Word document. There will likely be a number of things that don’t come out quite right, especially formatting, but it usually gives a good first draft that you can edit and save as a .docx file, which is what RAB requires.
I normally do a lot of simplifying of formatting when I’m preparing a .docx file for RAB anyway. From my experience, you just want to have a limited set of styles (often just 2 or 3) for a file that you import into RAB.
We do NOT want a PDF import functionality in RAB. Android devices can already read PDF files, if you want people reading exactly that format. It’s better to think of RAB as providing another way to present the data of a document, but as with other cases of different presentation forms (print, web page, app), you should not expect the formatting to be identical - it needs to be adapted to the medium.