5 Excellent Alternatives to Adobe Acrobat Reader DC

Portable Document Format (PDF) is a file format created by Adobe Systems in 1993 for document exchange. The format includes a subset of the PostScript page description programming language, a font-embedding system, and a structural storage system.

Zathura Highly customizable and functional document viewer
Evince Multi-page document viewer
MuPDF Lightweight PDF and XPS viewer
okular Universal document viewer with support for advanced document features
qpdfview Tabbed document viewer. It uses Poppler for PDF support