If someone scans the pages of a document in the wrong order then the resulting PDF file will have its pages in reverse order. On Linux this is very easy to fix. First install pdftk (if necessary):
sudo apt-get install pdftk
pdftk file1.pdf cat end-1 output file2.pdf
where file1.pdf is the name of your input file and file2.pdf the name of the ouput file.
(Thanks to emilien at Stack Overflow)