An update for the revised question...
#!/bin/bash
label="$1" #the label to be added to the footer
from=$2 #the starting number
#for each pdf in this directory
for input in *.pdf; do
#compose an output file name
output=${input%.*}.numbered.pdf
#count the pages
pagenum=$(pdftk "$input" dump_data | grep "NumberOfPages" | cut -d":" -f2)
#calculate the last page no
to=$(($from+$pagenum-1))
(echo -e ".nr FM 0.125i\n.ds CH"; for s in $(seq -f "%05g" $from $to); do
echo -e ".ds RF $label $s \n.bp +1"; done) |
groff -ms -Tpdf |
pdftk "$input" multistamp - output "$output"
#get ready for the next file
from=$((to +1))
done
Compose a basic groff -ms document with empty pages except the right footer .ds RF containing the desired text $label $s where $s is the page number which I padded with 0's in seq -f "%05g"
groff -ms macro applies a page header which is suppressed with .ds CH and .nr FM 0.125 reduces the footer margin height, so each "page" is just the footer with a page break .bp +1
Piping this text into groff -Tpdf creates a pdf which is then piped into pdftk via stdin - and, multistamped onto the input file and finally written to the output file.
My apologies to anyone who actually knows how to use groff properly if this is a hack .... I never even looked at it until today.