What are some alternatives?
calibre - Ebook manager, viewer & converter
Document Viewer - Document Viewer is a highly customizable document viewer for Android. - SufficientlySecure/document-viewer
Amazon Kindle - Amazon Kindle software lets you read ebooks on your Kindle, iPhone, iPad, PC, Mac, BlackBerry, and...
Xodo - Xodo is the ultimate cross-platform PDF viewer and annotator. And it’s 100% free.
FBReader - FBReader is an e-book reader for various platforms. Features:
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.