Answered
Two questions:
When a user first encounters a file in the .pdf format, does the OS installed on the machine automatically prompt the user to download and install Adobe Reader (specifically), and lead them to the download page, or is the file type just classified as "unrecognized," without further information?
I'd also like to verify that if for some reason a user encounters a "smart" pdf created with Acrobat Pro, which contains external links to other .pdfs available on a CD, it will still be functional even if the user is using some other program to view the .pdf.
Thank you so much for your time!
However if the OS has no software at all, it won't automatically direct people to install Adobe Reader - in Windows for example they are sent to search for suitable software online. They are far more likely to choose Adobe software given the high profile of Reader and Acrobat, but it's not guaranteed and some users have specific objections to using particular brands.
If a PDF contains real hyperlinks (and not something that uses scripting) then most third-party reader apps will follow them just as Adobe's do. There are of course many limitations to non-Adobe apps with respect to multimedia, scripting, forms, security etc. so in some cases only Adobe software will be able to even open the file in the first place, but hyperlinks have been part of the PDF specification for a long time, and so are well supported.