Doi is the main one. Most published papers and many other things have them. They're newer so often older things hang around too, I'd wager most new pubmed articles have dois too. There are several registries, crossref is the big one in the west but it's not the only one. They have probably the best access to the data out of all of the larger registries though. Dois are pretty good, though not persistent and... - Source: Hacker News / about 2 months ago

I managed to download it from researchgate.net. Send me a DM if you want me to send you the PDF. Source: about 1 year ago

Academia.edu, researchgate.net, surprising amount of stuff out there. Hopefully you have. A library or institution to help you gain access if PDF not freely available. Source: about 1 year ago

Also, their assertions about publications are kinda hinky. I note that the "published articles" they cite are all links to researchgate.net rather than the actual journals, and the one DOI I tried didn't work, for a paper supposedly published in January. Google Scholar also has no record of that paper (Fossils on Mars? A “Cambrian Explosion” and “Burgess Shale” in Gale Crater?) other than the researchgate link. Source: over 1 year ago

So I looked around and pubmed didn't offer anything interesting though I found something on researchgate.net. Source: over 1 year ago

Check out researchgate.net. Madami dun and mostly free to access. Source: over 1 year ago