From the Europe PMC Blog:
Europe PMC has been indexing preprints since 2018, hosting content initially from bioRxiv, ChemRxiv, PeerJ Preprints, F1000, and subsequently adding content from medRxiv and Beilstein Archives. Europe PMC now indexes over 100K preprints. Through a recent redesign of the search functionality, Europe PMC highlights more clearly search results that are preprints (versus research articles or literature reviews, for example).
However, an open question has been how to deal with the many different versions of a preprint appearing in the search results. The aim was to identify all versions of a preprint, link them together and offer a collapsed aggregate of these versions to users.
To this end, Europe PMC has improved the presentation of preprint versions and displays only the most recent version of a preprint in the search result page. Europe PMC links preprint versions based on their DOIs, allowing the user to access previous versions along with the edits, comments and citations associated with each specific version. From each preprint version the user can also link to the peer-reviewed, published version of the article if it is available in Europe PMC.