convert -thumbnail x400 -delay 90 3dmcmc_eccv2010.pdf eccv10_paper.gif
To create a thumbnails for all pages in the PDF document (say test.pdf which has 3 pages), do

convert -thumbnail x300 test.pdf test.png
ls
> test.pdf test-0.png test-1.png test-2.png

The “x300? tells the “convert” tool to keep the height at 300 pixels and modify the width to suit the height. If you have absolute dimensions in mind (say 400×300), do

convert -thumbnail 400x300 test.pdf test.png
If you want to control the width and have the height suited, do

convert -thumbnail 400 test.pdf test.png
What do you do if you want only the first page’s thumbnail? do,

convert -thumbnail x300 test.pdf[0] test.png
A neat feature of the “convert” tool is creation of animated gif’s having all the pages from the PDF document. do,

convert -thumbnail x300 test.pdf test.gif
“convert” is smart enough to figure out the output format from the file extension you provide in the arguments.

Nifty tools like “convert” from the ImageMagick suite are one of the reasons I love Linux! (ImageMagick is cross-platform, I’m referring the to utility of command-line tools and the UNIX philosophy of specialized tools that do one task really well)