It's arXiv preprint search & PDF viewer app.
All features are completely free to use.

arXiv.org e-Print archive
https://arxiv.org/

URL scheme:
preprint://

Note:
- This app uses arXiv APIs.
- This app is run by an individual and has nothing to do with arXiv.

Thank you to arXiv for use of its open access interoperability.

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* arXiv

arXiv (pronounced "archive"—the X represents the Greek letter chi [χ]) is an open-access repository of electronic preprints and postprints (known as e-prints) approved for posting after moderation, but not peer review. It consists of scientific papers in the fields of mathematics, physics, astronomy, electrical engineering, computer science, quantitative biology, statistics, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository before publication in a peer-reviewed journal. Some publishers also grant permission for authors to archive the peer-reviewed postprint. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, and had hit a million by the end of 2014. As of April 2021, the submission rate is about 16,000 articles per month.

Quoted from:
https://en.wikipedia.org/wiki/ArXiv