In general, ePub files will provide a better overall reading experience, because text will re-flow and can be re-sized on different devices. However, many of our publishers do not have ePub files available at the pre-publication stage, when titles are added to NetGalley (titles are typically added to the site 3-6 months before publication, although this is not a requirement). In that case, PDF files can be used.
For fixed layout titles, PDF will work best for now. Fixed-layout EPUB files cannot be opened on all devices. For Kindle, we’ve
found that fixed format mobi files will work on actual Kindle devices, but
there are issues with those files on the Kindle app for Apple devices. We’re
working to move to ACS5, which will address the ePub issue we’re seeing for
most devices (for which we use Adobe DRM) and are continuing to look into the
issue with the fixed format mobi files of Kindle apps.
It's helpful to remind readers, either in the front matter of the PDF or in the NetGalley "Notes" section, that they are receiving an unpublished, "not final" version of the material for professional reading purposes.