Decrypting a file

When your PDF file is password-protected, you can use xpdf to open it. Still it will not save it without the password which is annoying.

To do this, the solution I have found is to convert the file to postscript and then back to pdf. Note however that you may loose some information in the process.

You will need poppler (for pdftops) and ghostscript (ps2pdf) for this to work. Assume you want to decrypt the file secret.pdf which has the password foobar, then run:

$ pdftops -opw foobar secret.pdf
$ ps2pdf secret.ps && rm secret.ps

Merging several files

To concatenate several files into just one, just run:

$ gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=out.pdf -dBATCH $file1 $file2 ...

Where gs is provided by ghostscript.

Extracting only some pages

To extract only from page $start to page $end, of your document $input, and create a new pdf $output with those pages, run:

$ gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER \
     -dFirstPage=$start -dLastPage=$end \
 -sOutputFile=$output $input

My source for this trick is this tech tip article from Linux Journal.