Most PRO files are profiles, or similar settings and these cannot be exported as PDF documents. PRO files that are texts or graphics can be of course printed to a PDF file, but there is probably no dedicated PDF converter for that.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.