GIMP :: Editing Image PDF Files?

Import a pdf-file that only contains (scanned) images [book pages]. Rotate them (okay I can do this). Seperate and/or crop them (I can do this but my way is so complicated). Export back into a single pdf file. (I can't do this).

Usually I have a scanned book. So sometimes there are two pages on one image. To have an easier readability I like to seperate those. So what I do is:

- I cut/crop the page out of the image with the two pages

- I place that in a new layer

- I rotate that layer if needed (I use the transform tool, but it is a bit difficult to tell if the text is perfectly rotated, in Photoshop there is an automatic tool, is there one in gimp?)

- I go on with the next page

Problem is that layers are not of the same size. So I have to drag them around to have them (layers) all centered. Also image quality is much lower after importing it to gimp. How do I figure the original resolution of the image saved in the pdf? When I have them all in new layers exporting them to pdf will always result in one single page with only the first layer.

So is there an easier way to do this? Also why does exporting not work?

I'm using Gimp 2.8 on Ubuntu 12.04.