Depending on how your PDF was created, you are correct that PDFs can be much more space efficient than TIF images. The PDF file format is extremely flexible, and one of the ways it can be used to represent a document (especially if it is composed of simple lines and text) is as a series of drawing commands to render that text. Imagine in a worst case scenario, you have a blank page with the word TEST in the center. The PDF can be written so that it contains a single drawing instruction, draw "TEST" at those coordinates... maybe a 1k file when you include some of the overhead for the file header and other necessary structure. When you convert it to a TIF, you are actually then storing all the pixel data for each point on the page. (For example, 8.5in * 11in * 200pixels/in * 32bit/pixel = 600kb). However, a PDF could also just be a pure image on the back end (for instance, if you export a TIF document out of Laserfiche as a PDF, it just inserts the image into a PDF wrapper and you get no space-reduction from any clever drawing commands).