Download of SnowyOwl 2.3.1 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. This software for Mac OS X was originally created by Digital Bear Technology Limited. SnowyOwl for Mac lies within Productivity Tools, more precisely Office Tools. We recommend checking the downloaded files with any free antivirus.
From the developer:
SnowyOwl is a user-friendly tool to help you organize your research, or any other kind of dataset. It supports multiple libraries, and provides groups, tags, keywords, etc. to help you organize your dataset. You can easily sync your library, by putting it into your favorite web drive, such as iCloud, OneDrive etc. PDF annotations are automatically extracted, and your document's content is searchable.
You may want to check out more software for Mac, such as 3dissue, Blurb Tally or Pads X, which might be similar to SnowyOwl.