This is a problem caused by Academia.edu generally being very slapdash about bibliographic metadata. Even if some Zotero angels help you download the PDF, the metadata would most likely be incomplete. Essentially they're a walled garden with no interest in promoting views or downloads offsite, so they make this hard. The site is best avoid for serious scholarly purposes, but if you're a member there your best bet is to request a fix from them.

This is not a "simple task". Please reread @mark above and also the linked post. It is the academia.edu site that blocks Zotero from downloading the pdf in a normal way. This is _by design_ from the people at Academia. The items on academia.edu 1) do not contain metadata at all, 2) contain incomplete metadata, or 3) contain incorrect metadata.

I curate an online bibliographic database and have had several requests from authors to index their work that is on academia.edu. (SafetyLit indexes selected items on ArXiv and similar resources.) Without acceptable metadata and proper stable linking structures, indexing isn't feasible. Some time ago I wrote to Academia.edu with a few comments and requests to enable me to index material on the site. At first I received no reply. After writing again I received a snarky-toned reply that said they received my first message and ignored it. The essence of their email said that the company doesn't make it easy for Google to "cannibalize their content" so they certainly don't intend to go out of their way to help me with my service. I replied that SafetyLit is a free service without advertising and the same accurate metadata useful to Google Scholar would be useful to anyone who wanted to cite Academia.edu contents or index it properly. I said that good metadata will draw visitors to their site. I didn't receive a reply.

[it isn't irony]

See https://forums.zotero.org/discussion/42512/academiaedu-translator/