eleonorang Good tip! I also pinged Trados on Twitter to see if they could give you a hand, hopefully they'll get back to you ?
A question, though: why would you want to send back a PDF to the client? Normally, a simple Word document would be enough, unless you want to punish them for sending you the PDF in the first place ?
Normally you can use lots of converters from DOC to PDF. Here's the one I use: https://smallpdf.com/word-to-pdf