Adobe PDF specification was changed several times (See PDF History). These changes correspond to Adobe Acrobat product update releases.
- 1993 – PDF 1.0/Acrobat 1.0
- 1994 – PDF 1.1/Acrobat 2.0
- 1996 – PDF 1.2/Acrobat 3.0
- 1999 – PDF 1.3/Acrobat 4.0
- 2001 – PDF 1.4/Acrobat 5.0
- 2003 – PDF 1.5/Acrobat 6.0
- 2005 – PDF 1.6/Acrobat 7.0
- 2006 – PDF 1.7/Acrobat 8.0
- 2008 – PDF 1.7, Adobe Extension Level 3/Acrobat 9.0
- 2009 – PDF 1.7, Adobe Extension Level 5/Acrobat 9.1
At this time, the PDF format version 2.0 is in the development with the release date planned to 2012. There are a number of alternative modifications of the PDF format (for example, CompactPDF) made by independent developers.
You can learn more about PDF specifications here: Adobe reference for developers: http://www.adobe.com/devnet/pdf/pdf_reference.html
Adobe PDF specification version 1.7: http://www.adobe.com/devnet/acrobat/pdfs/pdf_reference_1-7.pdf
Portable Document Format reference version 1.2: http://www.pdf-tools.com/public/downloads/pdf-reference/pdfreference12.pdf
Quick review of PDF by Adobe Inc: http://www.adobe.com/devnet/livecycle/articles/lc_pdf_overview_format.pdf