Yes, it is 100% legal. Long answer: You are editing data on your personal memory card. You are not distributing the game ROM (ISO) or Konami’s copyrighted code. Option files are save files—the same as a custom roster in Madden or FIFA. However, downloading an option file that includes a modified SLUS_217.88 executable (a patched ELF file) enters gray territory. Stick to pure *.max or *.npo files to remain safe.
Looking for a PES 2008 PS2 option file? Here's a short, shareable post you can use on forums or social media: pes 2008 ps2 option file
A high-quality option file for PES 2008 typically addresses these areas: Licensing Fixes Yes, it is 100% legal