Excluding resources when building

I’m using several plugins bought from Asset Store.
There are many files such as documentation (pdf, etc.) included in those.

Should I delete them before building to reduce build size or Unity will automatically exclude those?

No need - if they’re not used in your game, they won’t be included in your build. The only exception to this is files in the /Resources subfolder, which are always included.

Since 5.3.2 you can exclude files and folders from build with “~” suffix.
See 5.3.2 Release Notes (687655), and the updated Unity - Manual: Special folder names.