Hi,
There are two options to solve this
option 1: use the "script" i.e use shell script , activity to decrypt the file in the workflow before importing file into adobe campaign.
option 2: you can follow the following steps:
Step 1:
download pgp either from https://sourceforge.net/projects/ppgp/ or https://pgp.en.softonic.com/
step 2: install the pgp
step 3: Use Data loading (file) activity in your adobe campaign workflow:
step 4: choose pre process the file option: refer to screen:

Step 5: Use the following command:
<path-to_pgp_if-not_global_or_server/>pgp.exe --decrypt --input nl6/var/vp/import/filename.pgp --passphrase "your password" --recipient recipient @email.com --verbose --output nl6/var/vp/import/filename
step 6 : save and execute the workflow.
Regards,
Amit