I haven't used Zinio in some time, but I seem to recall that some magazines supported reflowable text while others didn't. New Scientist comes to mind for the former. Analog was a thorn in my side because it didn't (and it is one of the least page dependent publications out there). You may want to check the publications that you are interested in while using the Zinio app to see if they support that option since, as far as I know, it is the only reader for capable of accessing publications purchased through Zinio. Since your library is paying for, I wouldn't complain too much.

As for those fixed format magazines being published in a PDF-like format, that seems to be exactly what they are. Poking around the directories revealed a bunch of, presumably, encrypted, PDF files.