The Google Books application, which, until recently, only e-books from the Play store open could be, let it be in a new version now also allows for epub and open pdf files of up to 50MB per piece.
There are some limitations to sideloading of titles. Google Books supports a maximum of 1000 files, each up to 50MB. Nevertheless, keep the app cross-platform a number of settings for each open file. Thus, page position, bookmarks and notes between, for example, an Android phone and an iPad kept. Moreover, the titles within Google Books and also through the browser to read.