Blank out part of PDF file

cmdline

## Problem: how to produce a PDF that erases a rectangle in a page of an input PDF (without rasterizing)?
## I will assume for example that we need to erase the rectangle with origin at (5mm,148.5mm) and size (200mm,143.5mm)
## in page 42 of `input.pdf` and save to `output.pdf` - maybe also assuming that all pages are A4.

## NB: this is for printing only.  Text below the rectangle WILL NOT be erased, merely obscured!

# Use Cairo Perl module tu create PDF file with white rectangle at specified position and dimension:
perl -e 'use Cairo; $surf = Cairo::PdfSurface->create("white.pdf", 210/25.4*72, 297/25.4*72); $cr = Cairo::Context->create($surf); $cr->rectangle(5/25.4*72,148.5/25.4*72,200/25.4*72,143.5/25.4*72); $cr->set_source_rgb(1,1,1); $cr->fill; $cr->show_page;'

# Use pdftk to extract pages before, at and after page 42:
pdftk input.pdf cat 1-41 output before.pdf
pdftk input.pdf cat 42 output tostamp.pdf
pdftk input.pdf cat 43-end output after.pdf

# Use pdftk to stamp the page with the white rectangle:
pdftk tostamp.pdf stamp white.pdf output stamped.pdf

# Reconstruct the PDF with from the parts:
pdftk before.pdf stamped.pdf after.pdf output output.pdf