Speaking for myself I really would appreciate if we could add pdfs manually to our Keil projects. I don't find the books tab very handy, the default datasheet and summary have to be downloaded from online and as these uC manuals are >1000 pages it is rather annoying to wait for it to download each time.
...If we could do away with the books tab and just allow for a folder to be added to the Project tab and let all PDFs added to a "PDFs" folder be ignored by compiler that would be best in my opinion.
Ah, you're right - it doesn't work!
:-(
You can specify a Custom Translator to be used during the build, but you can't specify a custom "viewer".
Personally, I would just have the PDFs (or, maybe, shortcuts to them) in the project folder, and open them directly with the Acrobat reader in the usual way; I don't feel the need to have every last thing (mis-)managed from within the IDE - whether it's uVision or anything else.