Use ImageMagic’s convert program to convert between image formats as well as resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.
To install ImageMagick in your ubuntu(or other debian distros) type this at terminal:
$ sudo aptitude install imagemagick
Convert PDF files (examples)
convert from pdf to jpeg
$ convert input_file_name.pdf output_file_name.jpeg
The above may give you a small image, For a large image use:
$ convert -density size input_file_name.pdf output_file_name.jpeg
replace size with your desired resolution eg: 200
convert from pdf to tiff
$ convert file_name.pdf file_name.tiff