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JPEDAL (Java Pdf Extraction Decoding Access Library)
An library for rasterizing and extracting content from PDF files in Java. [Open source, LGPL]
www.jpedal.org/

Jaws PDF Software
Developer of PostScript interpreters and PDF software tools and technologies to enable you to convert, write, and deliver PostScript, HTML, images and other documents. PDF creator and server available.
www.jawspdf.com/

PDF Research Companion
A designer's guide to the Portable Document Format. Provides links, resources, articles, case studies and more on Acrobat, Capture and FrameMaker.
www.performancegraphics.com/

The PDF Corner
Overview of the format, troubleshooting, application support.
www.prepressure.com/

Affordable PDF Tools
Merge, split or alter PDF files.
www.a-pdf.com/

PDF.Net
A cross-platform PDF library used to read, write and edit PDF documents. Available as a .Net component and as a C++ library. By pdfTron.
www.pdftron.com/

Siberix PDF Library
Create industry standard PDF files.
www.siberix.com/

Panda PDF Generator
Panda is an open source PDF generation API.
www.stillhq.com/

Search Adobe PDF Online
Search the web for PDF files. Web-based PDF files can also be submitted for inclusion in the search.
searchpdf.adobe.com/

PDF Research Companion
Description of PDF, how-to instructions for its use, additional resources supporting PDF, a comparison of PDF to PostScript, and links to related sites.
www.performancegraphics.com/