Q: Could there be copyright issues if a published paper is presented at conference or hosted online?

Copyright concerns are typically associated with republishing the same work/illustration in another journal or making information available whose copyright is with the publisher. It is however quite acceptable to present published work at a conference. While some poster presentations are published as proceedings in a journal, many are not. Further, since the mode of presentation is different, and the poster is not published in a similar manner as an article, there should not be any copyright issues.

For pdf files in ResearchGate, these should ideally be only open access articles that are readily available. ResearchGate has some basic information on copyright issues on their website.  Articles made available on their website should be articles that the authors have rights to make available, that is, articles that are open access. You are therefore correct in thinking that as long as the article is published open access, they can be made available through ResearchGate. However, this was not always followed and some concerns have been raised against ResearchGate, post which they have agreed to restrict access to copyrighted papers.

Related reading:

• Can we present a research paper in a symposium based on our published research work?
• Unintentional copyright infringement by an author: A case study