Do you also regularly experience difficulty obtaining the PDFs of publications? Either because they are behind a journal´s paywall or just not available online as PDF? There are several alternative ways to obtain these publications.
Let´s assume you just searched PubMed for interesting articles on your favorite subject. PubMed contains over 33 million biomedical publications, including books, so you are likely to find many interesting articles there (see my blog on search strategies). From PubMed, you can often click onto a journal´s site to obtain the article. If you are working at a research institute many of these will be freely available to you as your institute will have a contract with many of the major publishers. However, if you do not have institutional access, or if the journal is not covered under one of these deals, you will be asked to pay a fee. These fees can be per PDF (e.g. $35), or a fixed amount for (un)limited access to articles in that journal.
To avoid the frustration of having to pay you could opt for only searching PMC, which is similar to PubMed in that it also contains many biomedical publications, just fewer (only around 8 million) than PubMed, with the difference that it only contains the articles that are freely accessible (open access). There are however also other ways to obtain the PDFs you need.
When publications are not available to you in PubMed or PMC
After hitting a paywall in PubMed you may want to search Google Scholar. If you find the article there it will have a link next to it in case it is available somewhere as PDF. If it has an HTML link next to it this either means the full article is available as an HTML file or that you will be led back to the paywall of the journal.
Personally, I prefer to go to Research Gate which is a free online network for scientists to share publications, ask or answer questions, and that is used by over 20 million researchers. Here it is very easy to request a PDF from one of the authors, as all are happy to share their work. If none of the authors happen to be on Research Gate you can always send an email to the corresponding author directly to request the PDF. This may just take a bit longer. You can usually find the email address of the corresponding author on PubMed, under ´Author Information´ above the abstract. If it is not there, look for the email address on another publication from the same author.
Another website that shares links to scientific publications is Free Full PDF. It is a search engine that crawls all sorts of websites for PDFs, among others from institutions and labs as these often share their own publications on their websites. It contains over 80 million documents and is quite useful.
Another website that legally shares scientific publications that are available as open access is Unpaywall where you can download articles if you know the DOI. You can either use their online tool which will process up to 10,000 DOIs at a time, or use an extension to your webbrowser (check the website for details). This website claims to contain more than 31 million publications.
If you are not in a hurry or still have not found the publication you need you can always ask the librarian at your medical/institutional library to find the PDF for you. They will have access to other sources and libraries or can obtain the PDF from a journal at a discount. You often need to pay for this service though.
Ask the community
Another option is that you ask for the PDF via twitter, this works by tweeting the article title, its DOI, and an email address (replacing the @ by (at) to avoid getting spammed), and the hashtag #ICanHazPDF. Someone with access to the article might send a copy via email. Once the article is received, make sure the tweet is deleted (so people are not thinking you still need it). I assume this works especially well if you have lots of people from your own field among your twitter followers.
Illegal access to publications
Just for educational purposes, so I don’t say you should do this, but many articles that are not open access are also available illegally. Don´t break the law!
There is a website, Sci-Hub, that shares PDFs. The website claims to contain over 88 million publications. Very few articles are not available there. When one Sci-Hub domain is blocked there are usually alternative domains accessible.
Let me know in case you found any other good sources so I can share them here.