I have quite a few questions in mind that would work much better if I could attach PDFs of academic papers to the post. I could then refer to the PDF in the question. I have access to the PDFs through my university, but otherwise, anyone wanting to access the PDF would need to take out a subscription to that journal.
Is it possible to attach PDFs of papers to questions?
If no, is it possible include screenshots of specific parts of a paper?
Most likely those papers are copyrighted, and if you posted a downloaded version you would be violating that copyright as would SE. It is fine to reference papers and link to their hosting site. You can also quote from them or post a screenshot of a part of them as fair use.
Legal/ethical issues aside, if someone has to read all, or most, or several dispersed parts of a research paper in order to answer your question, it's less likely to attract interest, & therefore answers, as well as other readers who might be helped by it. Better in this case to extract & summarize relevant information, referencing the original paper for background.
This started out as a comment but once it got toward filling up three comments worth of text, I figured I better move it.
In response to whuber's and gung's points about 'fair use':
'Fair use' as a legal term applying to copyright seems to be part of US copyright law.
Copyright conditions can vary from country to country; for example Australian copyright law* is a little different - there are a list of somewhat similar exceptions to what would be exempt in the US, but the broader notion appears to be significantly more restrictive (there's no explicit and broad 'fair use' doctrine as there is in US law; on the other hand, things seem to pass out of copyright somewhat earlier).
* I was very recently on the receiving end of some somewhat strongly-worded education from an Australian student of law on this exact topic ('fair use') in another forum on the internet. He was able to point me to the precise sections of the Australian law in that case (which IIRC don't actually contain the phrase 'fair use' at all).
Not everyone is in the US, so there's something of a potential legal minefield here.
One person might take an action that would be legal for them in their home country (reproduce some section of a document under the 'fair use doctrine'), but which induces another person to inadvertently break the law in theirs (such as by opening a linked document or viewing an image of a section of text they have no reason to anticipate is potentially illegal for them to have even a temporary copy of on their machine, but which copy is generated by opening the link to the document or image - or even just viewing the question, in the case of say imgur links in a question).
People may well open such a link/view such an image, close it (perhaps even after noticing they shouldn't have) and not realize there's still a (potentially illegal) copy on their machine, but for which certain organizations may well seek to pursue them vigorously for were it later discovered.
Given the vigor, even extreme prejudice with which some particular organizations (such as the RIAA and MPAA) seem to have famously pursued (in a number of countries) even quite minor issues such as people having copies of songs on a machine when they were unaware the copies even existed, or where they already legally owned a copy of the material, it may be that we need to be extra cautious about advising people what they can do.
At the least, I think we should take care telling people they can safely rely on a doctrine like 'fair use' that might not even apply to them.
[edit: On the other hand, I don't think we should tie ourselves overly in knots trying to accomodate every legal framework. Ultimately people need to take some trouble to be aware at least of what circumstances apply in their own part of the world and make their own informed choices. I don't know the OP's locale, but it might be reasonable not to assume he's from the US until we know otherwise.]
