From where have you attempted to download Reader? If from the Adobe site, have you provided the correct information about your OS?
If you open http://get.adobe.com/readerin Safari in 10.5.8, it should automatically ask you to download Reader X. If you are facing any problem, you can also use ftp link - ftp://ftp.adobe.com/pub/adobe/reader/mac/10.x/10.0.1/en_US/
This is English language reader.
If you want it for other languages, download version 10.0 from ftp site - ftp://ftp.adobe.com/pub/adobe/reader/mac/10.x/10.0.0/ and select appropriate language.
You can always update your reader by using Help -> Check for Updates in Reader menu
Comments and highlighting functionality is available in Reader X. It is not available in Reader 9
Thanks so much for this advice - worked perfectly.