It is of course legal, but it's possible OP got deceived into an annual subscription when they thought it was monthly, as discussed 7 months back on HN: https://news.ycombinator.com/item?id=30222165
also been tricked by them. if some services did not lock pdfs to only be opened in adobe reader, there is no way I’d touch anything adobe. Hate these practices and their updater and installer is extremely intrusive! Someone should run a poll for most hated software providers..
fwiw there are tools to remove the copy protection / "unlock" the pdfavailable on github, like this one[1] for windows and a number of them[2] for *nix based on qfed[3] or ghostscript