Question: When I export a transparent PNG image to a PDF for Print file, why does the transparent background fill with noise? Web and Mobi do not have this issue.
Exporting PNG for Print PDF
Pressbooks tells Prince XML what PDF flavor to use. Print PDFs use PDF/x-1 standard and part of that standard is that images are not allowed to be transparent.
Here’s a discussion about it from the Adobe Forums:
https://forums.adobe.com/thread/2369506
PDF/x standard are meant for exchange where all the PDFs information (styling/fonts) is embedded in the document, which is why Pressbooks uses it for the print standard. The only PDF/x variant that supports transparency is PDF/x-4.
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