I have some useful book collections (pdf, djvu ...etc), and I am wondering if it's ok to share them with Mathematics site users, as I also need some others from you. So is sharing download links ok here?
No. I'd strongly advise against it.
For some previous discussion see here. Technically by the DMCA what would happen is that
- You post content copyrighted by others on this site.
- Copyright owner sees it, and contacts SE.inc
- We remove the infringing post.
How various jurisdictions treat links, however, is a bit confusing. I prefer you don't drag this site into the muddy waters of copyright policy and possibly tarnish the image of the community.
Independent of the legal and copyright issues:
No, you should not post questions or answers with the sole or main purpose of sharing or requesting files.
Questions on this site should be about mathematics. "Who has a copy of Brown's Convex Algebra?" is not a question about mathematics, it is about finding a book. And "Here is a link to a PDF of Smith's Higher Group Theory" is not even a question. This would be the case even if the files in question were completely legal to share. (I am guessing yours are not, which raises even more objections as in Willie Wong's answer.)
You may include a link to a relevant file as part of a question or answer. If somebody's question is answered in a book or paper, and that reference is freely and legally available online, you can and should give a link to it. However, it is best to link to an "official" site rather than your own private copy which could disappear someday.
The community (here and at MathOverflow) has occasionally tolerated questions about obtaining books or articles if they are particularly difficult or impossible to get through usual channels in any format or at any price, e.g. "Does anyone have a copy of John Doe's unpublished 1947 preprint on reticulated splines?" Such questions are the exception rather than the rule, and I for one would prefer that they remain rare.
Write posts with the only propose to share books are not real mathematical questions, so the answer to your question is no.
However, I think it's ok to paste some parts of the book or even the whole book in some cases in order to help others to understand better the context of the question. I think we don't have to worry to much with this issue, see why:
- The book is allocated in another sites, it's impossible to Mathexchange has problems with illegal contents which doesn't belong to it. I can give a lot of examples of sites which has links of for instance youtube, depositfiles, mega, etc... when there is a complain of illegal material is made, the book of the allocated site is removed, not the link of the site which has the link.
- I think we don't have to worry to much about this, because EVERY post has problems with copyright laws in some degree, believe me. I think is impossible to learn and share knowledge with such an ancient law and if we want to destroy Mathexchange, we are doing the first step, being pro to this kind of law.
Be free to downvote me...