Last digits/letters often define packaging (not package type, but weather it comes in a roll on in a box or whatever) type. If you look closely in the datasheet, the difference will be pointed out somewhere. Generally, this won’t make a lick of difference for a hobby user - it’s really meant for mass-production companies who require specific methods of setting the IC’s into their machines.
The PA might refer to a different temp spec or something of that sort (I know some chips have a military variant that has different temp/pressure/etc standard and have a similar addition in their part number) - but to be sure look over the datasheet. Be smart tho - use cntrl-f to find the specific sections that refer to variations.
Also, it could be package variations as well. DIP/QFTP - should also be mentioned in the datasheet.
damn you crosh.