<p>Agreeing with <a href="https://stats.meta.stackexchange.com/a/6638/17230">User1865345's answer</a>, I'd say best practice is to:—</p>
<ul>
<li><p>Give a full reference, including name &amp; title. That's a fail-safe in case of link-rot, &amp; an aid to recognition.</p>
</li>
<li><p>Link to the DOI, or if there isn't one, somewhere like JSTOR, arXiv, PubMed, &amp;c. That reduces the risk of link-rot; &amp; spares people having to download a PDF file before they even know if they want to read the whole paper, in the (typical) case that the site provides an abstract, or some kind of preview.</p>
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</ul>
<p>You may still wish to add another link; though, as <a href="https://stats.meta.stackexchange.com/questions/6637/are-scihub-or-mega-google-docs-articles-allowed-on-cv#comment22729_6637">Sycorax points out</a>, links oughtn't to be essential to understanding a question or answer, so it's all right if the first link isn't immediately accessible to everyone. If you do, &amp; don't know if you might be infringing copyright, or somehow abetting infringement, consider the following:</p>
<ul>
<li><p>The law. IANAL, but I imagine you are infringing copyright if you make available a complete article &amp; you're not the copyright owner &amp; don't have their permission (either specifically or through a general license). I've no idea about linking to copyright-infringing material that someone else put up. (In either case I believe the copyright owner may issue a D.M.C.A. take-down notice.)</p>
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<li><p>S.E.'s current <a href="https://stackoverflow.com/legal/terms-of-service/public">Terms of Service</a>. The 'Acceptable Use Policy' says you mustn't &quot;post content that infringes on another’s intellectual property right, including [...] copyright&quot;. Don't know if 'posting' can mean 'linking to'.</p>
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<li><p>Current site policy: see <a href="https://stats.meta.stackexchange.com/q/5196/17230">Links to copyright material</a>. For articles, at any rate, the consensus seems to be that it's not for other users to bother themselves about anyone's possible infringement of someone else's copyright.</p>
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<li><p>Readers' sensibilities: it would be courteous to avoid <em>direct</em> links to potentially copyright-infringing material; better they land on a page that describes the content &amp; leaves them the choice. If some might consider a particular site disreputable, I'd recommend making the target clear in the link text.</p>
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