Hi, welcome to the forums.
To attempt to answer some of the questions:
- The sync target files are not really designed to be accessed manually and please do not be tempted to modify them as they contain metadata required for the sync functionality to work.
Ideally you just install another client at this foreign computer (you can use Joplin Portable if you want to put it on USB drive to avoid an actual client install). Otherwise there are some community made tools such as Joplin-Webviewer. - Have a look at the search documentation you can add filters to the main search box (the ones above your note list/F6) such as tag: or you can use the goto anything (ctrl/cmd + p) and use a # before the tag you want to look for.
- I'll have to defer to somebody else, it can display PDFs inline but I'm not aware of it being able to do OCR searching