Quite a nice, simple tool for creating collages is fotowall; it is available in the repositories as version 0.9-7, although there isn't a ppa for a more recent version and there doesn't seem to have been much activity on the github page. However, the original developer, Enrico Ros, does note at his site that two universities are working on some elements of a future version 1.

Nevertheless, fotowall is a very useful program for creating collages; many different canvas sizes and layouts can be used and the pictures added to that canvas can be resized and arranged in many different ways. Text can be added, for example, to the base of pictures or you can have separate text boxes anywhere on your canvas.

Special effects like shadow or blur can also be turned on or off for each picture. It is possible to control how the pictures overlap and which ones are brought to the foreground or sent to the background. If you select a photo you can even warp it so it appears 3d- just click a photo and then click the red dot you can see in the top toolbar (as in the screenshot), and move it off axis. I find that it's best to just experiment and see what's possible with the program.

You can save the canvas as a .fotowall document, which you can continue working on later, or click the export button at the top right of the screen and then save it as an image, a wallpaper, or print directly from the program. The full list of options can be seen in the screenshot below; you may notice that there is no option to export as pdf, but that's no problem as you can just use imagemagick and run convert myfotowall.bmp myfotowall.pdf and specify any options if you need to.