Any pictures can be saved as PDF document, provided you find an editor or at least a viewer with printing capability. Some programs may even offer a dedicated PDF export function, but a virtual PDF printer will be sufficient for this kind of conversion. So the real deal here is to find a program that supports .ppm pictures (portable pixelmap graphics).
Convert Portable pixelmap graphic to Adobe Portable document format.
We have found 9 software records in our database eligible for .ppm to .pdf file format conversion.
Microsoft Windows software - convert ppm to pdf on Windows
A free graphics and photo editor and authoring tool
Convert from ppm file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned ppm to pdf. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A complete image retouching program
A photo and graphics editor from Ability productivity suites
One of the best bitmap graphic editing program
Apple macOS / Mac OS X software - convert ppm to pdf on OS X
Mac version of the popular graphics editor
Linux/Unix software - convert ppm to pdf on Linux
GNU Image Manipulation Program
Multiplatform software - convert ppm to pdf on any platform
A vector graphics editor
Cross-platform batch image transcoder compatible with 500 picture image formats
On-line services software - convert ppm to pdf on web
An online graphics converter and resizer