Abstracts are available in BrowZine from two different sources: all articles where PubMed provides abstracts and some articles with abstracts included in CrossRef data (currently limited to abstracts using basic formatting).
Together these two sources include abstracts for tens of millions of articles!
In the BrowZine apps for iOS and Android, the abstract will be shown on the loading screen after you have tapped an article to download.
iOS:
Android:
Additional abstract sources and formats are being researched to provide additional coverage in the future.