Remove watermarks from pdf file

pdf

Is it possible to remove annoying watermarks from pdf file in Linux?

For example, every page contains text

This file was merged by application XY.

Best Answer

If the PDF isn't encrypted, this might be possible but can be very hard. I know of no application that does that, so you will have to create your own using a library for PDF manipulation. Of course this requires knowledge of the PDF standard.

Related Solutions

Possible to extract title and pagenum of each page in a pdf file

The following script will print the first line of each page of the PDF file passed as argument, followed by a space and the line number... It uses tools from Poppler (package poppler-utils on Debian or Ubuntu).

#!/bin/bash
if="$1"
pages=$(pdfinfo "$if" | sed -nre 's/^Pages: +([0-9]+)$/\1/p')
for ((i=1; i<=$pages; i++)) ;do
    printf "%s %d\n" "$(pdftotext -f $i -l $i -layout "$if" - | head -n 1)" $i
done

How to convert a scanned PDF with OCRed text to one without OCRed text

Here is how I would remove the OCR-ed text should I have to...

First, you need to know, that OCR-ed text in a PDF is not a layer, but a special text rendering mode. The following screenshot from the official PDF specification lists all available text rendering modes:

For more background, please see these answers of mine on StackOverflow:

Now for the procedure I envisage:

0. Make a backup of your original PDF file

'nuff said...

1. Use `qpdf` to un-compress most of the PDF objects

qpdf is a beautiful command line tool to transform most PDFs into a form that makes it easier to manipulate through a text editor (or through sed):

qpdf                       \
  --qdf                    \
  --object-streams=disable \
    input.pdf              \
    editable.pdf

2. Search for spots where PDF code contains `3 Tr`

All spots in the editable.pdf where there is 'invisible' (a.k.a. neither filled nor stroked) text is marked by an initial definition of

3 Tr

Change these to now read

1 Tr

This should make the previously hidden text visible. Glyphs will appear in thick outlines, overlaying the original scanned page images.

It will look very ugly.

Save the edited PDF.

3. Change `Tj` and `TJ` text stroking operators to 'no-ops'

Whenever a text string is prepared for being rendered, the actual operator that is responsible for doing so is named Tj or TJ.

Look out for all of these. Replace them by tJ and tj. This will change them into 'no-ops': they have no meaning at all in the PDF source code; no PDF viewer or processor will "understand" them. (Be careful not to change the number of bytes when replacing stuff in PDF source code, because otherwise you may cause it to become "corrupted".)

Save the PDF file.

4. Check how the PDF file looks now

The PDF should now look "clean" again. The renamed text operators do not have any meaning any more for the PDF viewer, nor for any PDF interpreter.

5. Use Ghostscript to create the final PDF

This command should achieve what you want:

gs                        \
  -o final.pdf            \
  -sDEVICE=pdfwrite       \
  -dPDFSETTINGS=/prepress \
   editable.pdf

This final step uses editable.pdf as input. It outputs final.pdf. The output will have removed all traces of text. The input still had the text, albeit in an "unusable" form, because the operator renaming. Since Ghostscript does not "understand" the re-named operators, it will simply skip them by default.