Thanks. So, it works like photoshop and illustrator?
Depends a LOT on just what you mean by "works like"...
Being familiar with photoshop my first answer is "no" so as not to set unfair expectations... Yet in a simplistic sense, the answer is "yes".
Probably the better way to think of it is that annotations (text/drawing/highlights/etc) act similar to objects in a Word document.
These sit in a layer (maybe more than one) above the base PDF document layer. You can then manipulate these objects (copy/paste/delete/move as well as change color/size/font/opacity/thickness) with the specifics of what you can do being dependent on the object (e.g. you can change font of text and thickness of a line but not the other way around). When saved, these objects/layers remain and can be manipulated in other sessions and (if supported) in other PDF manipulation applications (I believe, it's been a while since I've tried).
Unlike photoshop you cannot actually manipulate the layer(s) themselves, this is handled under the hood as you manipulate the annotation objects. There's no ability to create new layers, name them, turn visibility on/off, do layer masks, etc.
What you can do, because not everything supports PDF layers, is flatten the document which
does behave similarly to the same action in photoshop. You'd typically do this when sending the PDF to someone via email just to ensure all your annotations convey properly.
PDF expert also allows you to fill in PDF forms, which is something I've found handy from time to time. Receive a PDF form, fill it out and sign it on the ipad before emailing it back.
Hope that helps.