Andreas suggested doing the sign and encrypt in one step. What part of the decryption didn't work?
You are encrypting a file, producing a ".pgp" file. You then sign that file, producing a ".pgp.pgp" file. You then try to decrypt the ".pgp.pgp" file, but this is the signature file. By "decrypting it" you are removing the signature information and the output is the .pgp file, not your original .dlt file. You need to do a second decryption. Alternately just decrypt the ".pgp" file and not the ".pgp.pgp" file, or do a combined sign+encrypt as suggested in the first comment.