Hi
I know this link is to a commercial site but the description of 'Understanding PDF's' is very good and for newbies like me, its very helpful.
http://www.verypdf.com/pdfinfoeditor/flavor-pdf.htm
and some good explanations
http://www.verypdf.com/pdfinfoeditor/pdf-a.htm
http://www.verypdf.com/pdfinfoeditor/pdf-x.htm
Understanding PDF's and images
Moderator: peterZ