The easiest way how to export scanned .ptn images as PDF document, would be to use the original program that saved them in first place - PaperPort. With that, you can easily export and save your images as a .pdf file.
Convert PaperPort thumbnail images to Adobe Portable document format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ptn to pdf on Windows
A scanning software for Microsoft Windows
Convert from ptn file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.