Add text to pdf file

1. convert to ps:

gs -dBATCH -dNOPAUSE -dSAFER -sDEVICE=pswrite  -sOutputFile=file.ps file.pdf

2. edit ps:
Before showpage command add

initgraphics
/Palatino-Roman 14 selectfont   %select font and size
10 775 moveto % set coordinate in pt (size is defined in %%PageBoundingBox:)
(text to insert) show

3. convert back to pdf

gs -dBATCH -dNOPAUSE -dSAFER -sDEVICE=pdfwrite  -sOutputFile=file.pdf file.ps