Xpdf is the major competitor to the use of ADE on eBook Readers.
Xpdf is an open source viewer for Portable Document Format (PDF) files. (These are also sometimes also called 'Acrobat' files, from the name of Adobe's PDF software.) The Xpdf project also includes a PDF text extractor, PDF-to-PostScript converter, and various other utilities.
Xpdf runs under the X Window System on UNIX, Linux, VMS, and OS/2. The non-X components (pdftops, pdftotext, etc.) also run on Win32 (Windows) systems and should run on pretty much any system with a decent C++ compiler. It does not support DRM.
Xpdf is designed to be small and efficient. It can use Type 1, TrueType, or standard X fonts. Xpdf is licensed under the GNU General Public License (GPL), version 2
 Windows version
The XpdfViewer® library provides a PDF file viewer component for use in Windows applications. XpdfViewer enables any Windows developer to add PDF viewing capability to their application. It provides the developer with full control over the PDF viewer. This is a DLL for commercial use.
XpdfViewer features include:
- support for PDF hyperlinks
- text extraction
- text find
- region highlighting, including the ability to read highlight files generated by search engines
- CJK support - CJK fonts available for licensing
 XPDF utilities
Here are some other tools based on the Xpdf code:
- pdftohtml converts PDF files to HTML.
- SWF Tools includes PDF2SWF, which converts PDF files to SWF (Flash) animations.
- PdfSearch is a Python-based utility for searching PDF files.
- PDFKit is a framework for PDF tool development.
- Applets in PDF adds Java applet support to Xpdf.
- PDFtops will generate a PostScript file.
- pdftotext will generate a TXT file
- pdfinfo -- dumps a PDF file's Info dictionary (plus some other useful information)
- pdffonts -- lists the fonts used in a PDF file along with various information for each font
- pdfdetach -- lists or extracts embedded files (attachments) from a PDF file
- pdftoppm -- converts a PDF file to a series of PPM/PGM/PBM-format bitmaps
- pdfimages -- extracts the images from a PDF file
Command line options and many other details are described in the man pages: xpdf(1), etc.
All of these utilities read an optional configuration file: see the xpdfrc(5) man page.