Unpaywall vs Open Access Button vs CORE vs Google Scholar: Which Finds Free Papers Best?

You have the title or DOI of a paper, it is behind a paywall, and you want to know whether a free copy exists anywhere. Four tools are usually recommended for this job: Unpaywall, the Open Access Button, CORE and Google Scholar. They overlap, but they are not the same, and one of them no longer exists. Here is how they compare for finding a free copy of a paper you already know about, rather than for discovering papers on a topic.

Unpaywall

Unpaywall is run by OurResearch, a non-profit that also runs the OpenAlex catalogue of scholarly works. It keeps a database of tens of millions of free-to-read copies of articles, built by checking every DOI it knows against publishers, university repositories, preprint servers and archives such as PubMed Central. Every copy it lists has been made free legally, by the author, the publisher or a repository.

For readers, the main product is a free browser extension for Chrome and Firefox. Open an article on a publisher's site and, if a free copy exists, a small green tab with an open padlock appears at the edge of the page. Click it and you go to the free copy. There is nothing to search; the lookup happens automatically from the DOI on the page.

Strengths: very wide coverage, reliable version labels (published, accepted manuscript or preprint) and the data behind many library tools. Weaknesses: it works by DOI, so it cannot find anything for a paper without one, and the extension has to be installed and only triggers on pages it recognises.

Open Access Button

The Open Access Button was a lookup tool and browser extension from the non-profit OA.Works. You pasted a DOI, link or title and it searched the open sources; if nothing turned up, it emailed the author on your behalf asking them to share a legal copy and tracked the request.

It is now closed. OA.Works shut down the Open Access Button, its extensions and its related InstantILL service on 18 November 2025, and its own page recommends Unpaywall as the nearest replacement. You will still see it recommended in older library guides, so it belongs in this comparison mainly so you can stop looking for it. The one thing it did that the others do not, asking the author, you can still do yourself: our guide to emailing an author for a paper has a template.

CORE

CORE is an aggregator run by the Open University in the UK with Jisc. Rather than pointing at copies on other sites, it harvests full texts from thousands of repositories and open-access journals and stores them, so it has its own very large collection of free PDFs that you can search and read on core.ac.uk.

Its reader tool is CORE Discovery, a free browser extension for Chrome, Firefox and Opera. Like Unpaywall, it adds a marker to a publisher page when CORE holds a free copy. CORE says it can also match papers that have no DOI, by title and author details, which is useful for older papers, theses and working papers.

Strengths: a huge in-house collection, a usable search engine of its own and matching without a DOI. Weaknesses: copies come from repositories only, so a free copy on the publisher's own site may be missed, and the version of a paper is not always labelled clearly.

Google Scholar

Google Scholar is not an open-access tool, but it links to free PDFs wherever it finds them, including authors' personal pages, conference sites and lab websites that none of the indexes above cover. Search the paper's title and look to the right of the result for a link marked [PDF] or [HTML]. The "All versions" link under a result lists every copy Scholar knows about.

Strengths: the widest net of all and no installation. Weaknesses: it does not check that a copy is legal, so a result can be a copy the author was not entitled to post, and it does not tell you which version you are looking at. Our guide to finding free PDFs on Google Scholar covers its quirks.

Which should you use?

• For one-off lookups with nothing installed, search the title in Google Scholar, or use a finder that queries Unpaywall for you.
• If you read publisher pages all day, install the Unpaywall extension, and add CORE Discovery for the papers Unpaywall misses, particularly older ones without a DOI.
• For theses, reports and working papers, search CORE directly.
• If none of them finds a copy, the paper probably has no free version yet. Email the author, or ask a library.

Whichever tool you use, check which version you have found. A preprint may differ from the final paper, which matters if you plan to cite it; see preprint vs published paper.

How this site fits in

Our finder uses the Unpaywall and OpenAlex data, so it finds what the Unpaywall extension finds, without an extension and from a DOI, a link or a title. It labels the version of every copy and gives each paper a page you can share. If it finds nothing, it offers the legal routes that remain, including a ready-written email to the author. For a broader list of search engines, including tools for discovering papers on a topic, see the best free tools for finding research papers.