The file is part of your legitimate GTA V installation. If it is broken, use the feature in Steam or the Rockstar Launcher. This is the only safe, legal, and effective way to restore x64a.rpf without compromising your computer or game account.
A: If you use a modified or mismatched version, yes. The Rockstar anti-cheat system checks file integrity. Only the official, verified file is safe for online play. X64a.rpf Download
If you are a modder, learn to use mod folders (mods folder) so you never have to modify the original x64a.rpf again. Always keep a backup. The file is part of your legitimate GTA V installation