You don't have to use Photos -- if it's a PDF, say in an email, just open it. All the markup and signature tools are right there.

Of course it's a million times easier to do this on a desktop (or laptop), than on an iOS device.

@Greg A. Woods OK good to know. Yes, too true for a desktop - I tend to want to do all my stuff there rather as I have three monitors and can see what is actually going on.

and of course desktop/laptop pointer devices are far easier to manipulate than fat fingers on a small mobile screen!