That depends. What OS are you running? Here's a good standard...
If you are using Mojave, you should download 2.5.
If you are using High Sierra, you should download 2.4.1.
If you are using Sierra, you should download 2..4.1.
If you are using El Capitan, you should download 2.4.1.
Any OS older than El Capitan, you probably are going to need to use Paperless 1.