[warfangle]

Exporting to, say, XCF or a similar image file format with layers (is there one out there that will work across Photoshop+GIMP?) would be much more useful to a graphic designer than a PDF. PDFs are great for printing and overview, but having wireframes in a layered image file format for your graphic designer would be a huge plus.

[peter_severin]

You can also do PNG exports. There is batch export that will let you export a set of screens all at once. You'll have multiple files but I am not sure if it's a limitation.

[warfangle]

Layers, I think, are more important for individual sections of the wireframe - this allows the eventual visual designer to take portions of the design atomically, leading to greater internal visual consistency throughout the site. It also eases the pain of slicing and dicing ;)