A real example from a working archive:
PES_2008_PS2_EU_SLES54915_Option_File_Final.max
Contains: pes 2008 ps2 option file
Short answer: 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. A real example from a working archive:
Make sure the Option File matches your game disc region: PES_2008_PS2_EU_SLES54915_Option_File_Final
Note: If you have a PAL file and an NTSC game, the save will not work unless you convert the file using "PS2 Save Converter" tools.
If you just want a plug-and-play experience today: