The Internet is an excellent medium for sharing information. However one of the purposes of copyright is to allow the author to control how their work is disseminated. Since nearly everything shared online is covered by copyright this can create problems in determining how to share the interesting bits of information you discover online.
Downloading a copy of most files or web pages is easy. In a lot of ways its no different from printing them. However it is important to remember that downloaded copies are for your personal use. Once you begin emailing them or re-posting the material on another site you are ingaging in copyright infringement. Usually on a small scale and perhaps with a valid fair use defense, but not always. For example downloading runs or even issues of a journal and making them available is a clearly actionable violation of copyright.
Re-posting can also be a copyright violation. Once you have copied the work of another person and posted it somewhere outside of their control you have infringed. Small portions used for comment or journalism (of any sort) is easy to defend. The more unique value you add to the portions used the safer your defense will be. It is important to remember that materials kept behind a pay wall (i.e. most academic journal articles) are commercial products so smaller portions should be used or permission purchased from the copyright holder.
Linking is the easiest and most legal method for sharing found material. The person that finds the material transmits a link to their audience. If the audience has the permissions required to view the linked address they will see it once they click the link. If the audience does not have permission to view the material then they won’t reach it. The person or group that originally posted the content can remove it whenever they wish and have suffered no infringement. You may also be contacted to remove the link depending on the context that you are posting it and the amount of traffic your referral is generating.
In general linking is the best way to share information or documents online from a copyright point of view. If linking does not suit your needs then create your own content by copying small portions of the source document and commenting on them. Downloading and distributing the content itself is the most legally actionable method and should include a full fair use analysis. Also make sure you are downloading only materials that you have legal access to, otherwise you may find yourself with no legal defense for the infringement.
Remember the absolute best way to share content is with the permission of the copyright owner. Check the site to see if the material includes a list of permissions relating to how site content can be used.