There is no need to install a separate app or a browser extension, if you want to read epub files on your PC or Mac computer. If you have a Google account, you can do it using Google Play Books browser viewer.
Adding a file to a Google Play book library is easy.
Simply go to Google Books (make sure to be logged in to Google), select My Books, then Uploads. Click on a blue Upload files button and choose the documents from either your computer’s disc or Google Drive.
Once the file is loaded, simply click on its cover and it should open in a clean, double-page reader mode.
An extra benefit of using Google Play Books for reading files online is the ability to instantly translate text to any of the languages supported by Google Translate.
You can translate not only single words, but also longer passages of text. Again, no other browser extension is needed for that.
Obviously, you can open and read pdf files as well.
These files will be available on any other computer. You can access them via your Google account.
There are advanced browser extensions like MagicScroll on Chrome, but if you read epub or pdf documents occasionally, a dedicated tool might not be needed.
Also, if you don’t have to read Adobe DRM files (epub or pdf), and don’t have to authorize your computer with Adobe ID, there is no need to download Adobe Digital Editions application.
Although not exactly for that purpose, using Google Play Books platform is the most convenient way to open and read own epub/pdf files online.