gistfile1.m

// Add these two method declarations to the Parse.framework/Headers/PFObject.h header file in your Xcode project:

- (void)serializeToDataFile:(NSString *)filename;

+ (id)objectFromDataFile:(NSString *)filename;

// and you can start saving and loading PFObject on disk

PFObject *object = [PFObject objectWithClassName:@"Todo"];
[object setObject:@"Sample Text" forKey:@"text"];
[object saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
    [object serializeToDataFile:@"test.obj"];
}];

// Elsewhere ...    
PFObject *newObj = [PFObject objectFromDataFile:@"test.obj"];
NSLog(@"%@", newObj);

I'm not sure I understand the damn thing. Is it because they did implement the method serializeToDataFile and objectFromDataFile but didn't declare them in the header because they thought it wasn't ready ? Could you provide a bit more detail about this please ?

They are just considered 'private' methods, as in Parse does not intend for external developers to use it and it's not supported, could change, etc..