Apache POI is actually included in PRPC 7.x onwards (maybe even PRPC 6 - I can't remember): so it should be possible to use this library in an Activity (etc) to process a WORD file; create some sort of intermediate format, create some HTML, convert that using PRPC's HTMLTOPDF mechanism (which uses the third-party library : http://pd4ml.com/ for this).
Alternatively: you can do this sort of thing client-side - using a Print-To-PDF mechanism (outside of PRPC); possibly you could do this server-side as well if you can install a print-to-pdf Printer Driver onthe server and have PRPC 'print' to it ?
I haven't got any experience of this particular Flow Action "ConvertAttachmentToPDF", but I took a quick look at it : it seems to 'outsource' the WORD->PDF conversion to the 'pdm' (rather than doing the conversion within PRPC itself).
The History Tab on the Activity has the following note:
Send a word doc to pdm by inserting a request record
'PDM' here refers to a deprecated Pega Product called 'PegaDISTRIBUTION Manager' : this product is no longer provided by Pega.