No, that is incorrect. If the user has uploaded a copy of the PDF, we both show it on the page as an HTML 5 conversion and offer the PDF file as a download.
$ wget -O file.pdf http://www.academia.edu/attachments/27796552/download_file
--2013-01-13 12:55:35-- http://www.academia.edu/attachments/27796552/download_file
Resolving www.academia.edu... 50.16.203.88
Connecting to www.academia.edu|50.16.203.88|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://www.academia.edu/login?cp=/attachments/27796552/download_file&cs= [following]
--2013-01-13 12:55:35-- http://www.academia.edu/login?cp=/attachments/27796552/download_file&cs=
Reusing existing connection to www.academia.edu:80.
HTTP request sent, awaiting response... 302 Found
Location: http://academia.edu/login?cp=/attachments/27796552/download_file&cs=&tcw=1 [following]
--2013-01-13 12:55:36-- http://academia.edu/login?cp=/attachments/27796552/download_file&cs=&tcw=1
Resolving academia.edu... 50.16.203.88
Reusing existing connection to www.academia.edu:80.
HTTP request sent, awaiting response... 302 Found
Location: http://www.academia.edu/cookies_required [following]
--2013-01-13 12:55:36-- http://www.academia.edu/cookies_required
Reusing existing connection to www.academia.edu:80.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: “file.pdf”
[ <=> ] 27,784 --.-K/s in 0.1s
2013-01-13 12:55:36 (284 KB/s) - “file.pdf” saved [27784]Why?
Sure, at a personal level, it might be slightly annoying to you to have to create an Academia.edu account. But Academia's entire value is in the users, content, and community which have built up around it. If viral hooks like "signup to download" substantially grow the Academia community (and I believe this one does), then the value which they will create for you in having an accessible library of the world's research will far offset the small annoyance of having to sign up.
Don't put yourself before the content, that's the same mistake everybody else is making in this space.