I fix a lot of files free. It's a lot faster & easier than explaining why and I'd end up having to teach them how to do it anyway, which I don't have the time or patience for. Then you have to explain that, yes we actually do charge for this sort of thing, it's what we do for a living.
If it's something that will be really time consuming I'll charge, or give them the option. As a rule the biggest problem is dealing with 'graphic designers' who have taken a 6 month Photoshop program somewhere. They know everything, so you can't pull the wool over their eyes.