Xpdf is an open source PDF viewer for the X Window System and Motif. Xpdf runs on practically any Unix-like operating system. Xpdf can decode LZW and read encrypted PDFs. The official version obeys the DRM restrictions of PDF files, which may prevent copying, printing, or converting some PDF files. There are patches which make Xpdf ignore these DRM restrictions.

Xpdf includes several programs that don't require the X Window System, including programs which extract images from PDF files or convert PDF to PostScript or text. These programs run on DOS, Windows as well as Linux and Unix.

Xpdf is also used as a backend for other PDF reader frontends such as KPDF and GPDF, and its engine, without the X11 display components, is used for PDF viewers including BePDF on BeOS, !PDF on RISCOS and PalmPDF on Palm OS.

Xpdf exists also for AmigaOS. The Amiga version requires that in the system it should be loaded also a limited version of X11 engine called Cygnix.