Step 1: Assign each letter its position number, A = 1 up to Z = 26. XEROX becomes 24-5-18-15-24, and its code YFSPY becomes 25-6-19-16-25. Each number in the code is exactly one more than the original.
Step 2: Take PAPER and convert to numbers: P = 16, A = 1, P = 16, E = 5, R = 18.
Step 3: Add 1 to every number: 17, 2, 17, 6, 19, which correspond to Q, B, Q, F, S.
\[\boxed{\text{QBQFS}}\]