I want a few suggestion as to what is the best cloud based version control app for PDF files. In which site should I post my question?
Nowhere, other than https://softwarerecs.stackexchange.com/.
Asking for product recommendations is generally discouraged (more so on some sites than others) as it invites spam and link rot.
Search engines are designed to aggregate links to resources while excising worthless spam and keeping search results up-to-date. The best solution is to use search engines to conduct your research, identify the resources that appear to best suit your needs, then test them.