From the Europe PMC Blog:
From July 2018, Europe PMC will include abstracts for preprints that have a DOI and can be retrieved via Crossref metadata services. This means that about 37,000 preprints will be immediately discoverable in Europe PMC; a figure which we anticipate will grow by around 2000 preprints every month.
[Our emphasis] By restricting Crossref searches by DOI prefix, the initial preprint servers to be indexed include: bioRxiv, PeerJ Preprints, ChemRxiv, F1000Res, and the Open Research platforms powered by F1000: Gates Open Res, Wellcome Open Res, HRB Open Res, AAS Open Res, and MNI Open Res.
We are using this filtering approach in order to include preprints that have screening protocols in place, and to ensure we do not inadvertently include blog posts or other types of non-peer reviewed content. You can expect to find preprints in Europe PMC within 24 hours of being sent to Crossref; links to the full text on the preprint server via the DOI are included.
To distinguish preprints from peer reviewed articles in Europe PMC, each preprint is given a PPR ID, and is clearly labelled as a preprint, both on the abstract view and the search results . When preprints have subsequently been published as peer-reviewed articles and indexed in Europe PMC they are crosslinked to each other. About 14,000 preprints have so far been published in peer reviewed journals via these cross-link detection methods. All preprint content will also available in Europe PMC APIs as well as on the website.