Adding /Enforce viewer preference

[samuelbradshaw]

In the PDF 2.0 specification, there's a way to prevent a PDF from being scaled to anything other than 100% when printing (at least in Acrobat; other PDF viewers don't consistently respect it). This is done by by adding these two viewer preferences to the PDF:

/Enforce [/PrintScaling]
/PrintScaling /None

I was able to set /PrintScaling /None with writer.viewer_preferences.print_scaling = '/None', but I haven't figured out how to set /Enforce [/PrintScaling]. Is this possible? The closest I've gotten is /Enforce (PrintScaling). Here's my code:

from pypdf import PdfReader, PdfWriter
from pypdf.generic import NameObject, TextStringObject

reader = PdfReader('input.pdf')
writer = PdfWriter()

for page in reader.pages:
  writer.add_page(page)

writer.create_viewer_preferences()

# Add viewer preference: /Enforce [/PrintScaling]
# The code below doesn't work – it gives me /Enforce (PrintScaling)
vp = writer._root_object['/ViewerPreferences'].get_object()
vp.update({ NameObject('/Enforce'): TextStringObject('PrintScaling') })

# Add viewer preference: /PrintScaling /None
writer.viewer_preferences.print_scaling = '/None'

with open('output.pdf', 'wb') as f:
  writer.write(f)

See PDF 2.0 specification, page 430

[pubpub-zz]

This feature has been introduced in PDF 2.0, this is why it has not been implemented yet. I just note a small error :
vp.update({ NameObject('/Enforce'): ArrayObject([TextStringObject('PrintScaling')])})

can you confirm it is working?

I should be able to propose a PR

[samuelbradshaw]

Thank you! It didn't work:

vp.update({ NameObject('/Enforce'): ArrayObject([TextStringObject('PrintScaling')]) })
Produces this:
/Enforce [ (PrintScaling) ]
(I checked in Acrobat and Acrobat didn't recognize it as valid)

vp.update({ NameObject('/Enforce'): ArrayObject([TextStringObject('/PrintScaling')]) })
Produces this:
/Enforce [ (\057PrintScaling) ]
(Acrobat didn't recognize this either)

vp.update({ NameObject('/Enforce'): ArrayObject(TextStringObject('/PrintScaling')) })
vp.update({ NameObject('/Enforce'): ArrayObject('/PrintScaling') })
vp.update({ NameObject('/Enforce'): ArrayObject(['/PrintScaling']) })
These give me an error: AttributeError: 'str' object has no attribute 'write_to_stream'

This is what I'm hoping to get:
/Enforce [/PrintScaling]
(I've verified that this works in Acrobat – tested by manually editing the PDF in a text editor)

[pubpub-zz]

oups I missed the error:
vp.update({ NameObject('/Enforce'): ArrayObject([NameObject('/PrintScaling')])})

[samuelbradshaw]

Yes, that worked! Thank you!! :)

[pubpub-zz]

PR #2511 is under review to ease this