# Where to read research papers for free, legally

Most research papers have a free, legal copy somewhere: a preprint server, a university repository, a government archive or the author's own page. The fastest way to find it is to search by the paper's DOI with an open-access tool such as Unpaywall or OpenAlex, then try the sources below in order. You almost never need a pirate site.

## Start with the DOI

Every published paper has a DOI, a permanent identifier that looks like `10.1038/nature12373`. Open-access tools use it to find every legal copy of the paper at once, so copy it from the publisher's page before you start.

## Eight legal places to look

1. **Unpaywall.** A free browser extension that shows a green tab when a legal free copy of the paper you are viewing exists ([Unpaywall](https://unpaywall.org/)).
2. **OpenAlex.** An open catalog of over 250 million scholarly works. Each record lists its open-access locations, including the free PDF ([OpenAlex](https://openalex.org/)).
3. **arXiv.** Preprints in physics, mathematics, computer science, statistics, economics and more, all free ([arXiv](https://arxiv.org/)).
4. **bioRxiv and medRxiv.** Preprints in biology and medicine. Check whether a later published version changed the findings ([bioRxiv](https://www.biorxiv.org/), [medRxiv](https://www.medrxiv.org/)).
5. **PubMed Central and Europe PMC.** Free full texts of biomedical papers, including many funded by public money ([PubMed Central](https://www.ncbi.nlm.nih.gov/pmc/), [Europe PMC](https://europepmc.org/)).
6. **CORE.** Full texts from thousands of university repositories worldwide ([CORE](https://core.ac.uk/)).
7. **The author's own page.** Many researchers post their accepted manuscripts on their personal or university pages. Google Scholar's "All versions" link often finds them.
8. **Ask the author.** Email the corresponding author for a copy. Most are glad someone wants to read their work.

## Preprint, accepted manuscript or published version?

A free copy is not always the final one. A **preprint** is the version before peer review. An **accepted manuscript** has passed peer review but lacks the journal's formatting. The **published version** is the journal's final copy. For most reading, an accepted manuscript is as good as the published version; for preprints, check whether the paper was later published and whether anything changed.

## If you are at a university

Your library pays for many journals. Sign in through your library's website, use a browser tool your library recommends, or ask for an interlibrary loan for anything it does not hold.

## How Nerdi finds free PDFs

[Nerdi](https://nerdi.pages.dev/) is an iPhone app that shows three research papers a day. It reads each paper's open-access locations from OpenAlex and only labels a paper "Free PDF" when a legal copy exists, opening it right in the app. Its [field pages](https://nerdi.pages.dev/papers) list new papers with free PDFs in 20 fields.