Dropbox is a bad option if you intend to share it with too many people; pdfs are often large enough you'll get the "This account is generating too much traffic! Try back later" pretty quickly. Dropbox is great for sharing with the family or a small email list, but not for linking to the general internet.

Dropbox doesn't expose your PDFs to search engines, so it doesn't work as a public reference dump. (Github does, I think, but it's not so easy to use - can you imagine ordinary people trying to use git to expose documents to the world?)

Now imagine I know very little about computers and just want to upload and share a PDF file, i.e. click "upload" and get a link--an imgur for PDFs. I don't know of a solution like that.