Users of proprietary operating systems such as Microsoft Windows and Apple's Mac OS use this program to view files stored in PDFs, or Portable Document Format files. Users of Unix-like operating systems, free or proprietary, usually use programs such as xpdf or GhostScript to view PDFs; however, these programs cannot always handle the full range of PDF features. Raph Levien is working hard on adding full PDF 1.4 support to Aladdin GhostScript; versions of same are released after a delay as GNU GhostScript, licensed under the GNU GPL. I tried a beta of Acrobat Reader for Linux some time ago; it ate 180 megabytes of core before I decided that enough was enough, and sent it a polite SIGKILL.