Okular (the default standard document viewer on the KDE desktop) opens ePubs, PDFs, plaintext, and I'm sure other formats as well. It also has highlighting and

Okular (the default standard document viewer on the KDE desktop) opens ePubs, PDFs, plaintext, and I'm sure other formats as well. It also has highlighting and annotation capabilities. I'm quite sure every other operating systems' GUI environment has at least one similar app. I know MacOS has a "Preview" app built in that opens a zillion file formats in a read-only viewer.