Okular

Okular is KDE's universal document viewer. It opens PDF, ePub, MOBI, CBZ/CBR, DjVu, XPS, TIFF, and many more formats. Annotation tools let you highlight, underline, add notes, and stamp documents. Text-to-speech reads documents aloud. Free, open-source, and cross-platform. More capable than most built-in PDF viewers with a clean, modern interface. The document viewer you install when your OS default isn't enough.