Wouldn't it be nice if you had one program to view them all? That's exactly what Okular does. It's an open source universal document viewer for Windows, Linux and macOS. The program is made by KDE, a name Linux users should be familiar with, among other creations they are the ones behind the popular Kubuntu (Ubuntu + KDE Software) distro.
The Tools menu has some extra options such as the magnifier tool, and more importantly the annotation tool, which is called "Review". It allows you to add a note as pop-up, inline, highlight text, add a stamp, or to draw a line (freehand/straight), polygon, ellipse or a cool Typewriter annotation.
Calling Okular a document viewer is an understatement, because it can also be used to view images, though that isn't its primary function. Here are all document and image formats that are supported: G3, CHM, DDS, DJVU, DJV, EPS, EPSI, EPSF, EXR, FB2, GIF, XCF, HDR, PIC, JPEG, JPG, JPE, JP2, JPG2, MNG, MOBI, PRC, ODT, OKULAR, PBM, PCX, PDF, PGM, PNG, PPM, PS, PSD, RGB, TGA, ICB, TPIC, VDA, VST, TIF, TIFF, DVI, WWF, BMP, DIB, ICO, XBM, XPM, OXPS, XPS, CBZ, CB7, CBR, CBT, EPUB, DOC.
See Okular is an open source universal document viewer for Windows, Linux and macOS - gHacks Tech News
Okular is an open source universal document viewer for Windows, Linux and macOS. It can annotate PDFs, view images, extract text and more.