Good PDF resources (PDF internal structure)

http://www.webcheatsheet.com/php/reading_clean_text_from_pdf.php
http://www.gnupdf.org/Introduction_to_PDF
http://en.wikipedia.org/wiki/ITextSharp
http://www.1t3xt.com/products/itextsharp.php
http://www.codeproject.com/KB/graphics/iTextSharpTutorial.aspx
http://pdfedit.petricek.net/en/screenshots.html
http://www.accesspdf.com/pdftk/
http://www.windjack.com/products/pdfcanopener.html
http://www.glyphandcog.com/textext.html
http://www.glyphandcog.com/products.html
http://www.foolabs.com/xpdf/download.html
http://blog.amyuni.com/?p=552
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=IWS-Chapter04a#4d3110c8
http://en.wikipedia.org/wiki/UCS-2