We have PDFs on the internet that require 7.0.9. A few companies are still on earlier versions and do not allow their employees to upgrade reader. What percentage of internet computers are pre-7.0.9? What is the break down of versions? Adobe has Flash and Shock wave penetration posted but not reader.
I recommend that people use Acrobat 5 as the best version to write
against, UNLESS they have a reason to write against a more modern
standard. For example, if you want to have rights enabled pdf files,
Acrobat 5 won't cut it.