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Yes, what you describe as “flattening” of PDF files is a horrendous practice.
(1) Adobe Photoshop is not, repeat not, repeat yet again not a PDF file editor. Opening a PDF file in Photoshop results in all content being converted to a raster image at a single resolution in a single color space. You lose the scalability of vector and text objects.
(2) What you are doing does not “reduce the risk of a printing error” — sounds like an old wife's tale told by an ignorant, Luddite printer from the last century.
(3) For printing, the recommended PDF file format is PDF/X-4 with live transparency (i.e., not flattened) and with image in their original color spaces with ICC color profiles. Transparency blending and color conversions are best handled at the RIP/DFE itself. All reasonably modern RIPs can handle this without a problem. And even if you print a PDF/X-4 file to a PostScript RIP, Acrobat's printing will properly handle converting the PDF with live transparency and color-managed content into appropriate opaque CMYK PostScript without those stupid transparency flattening artifacts.
(4) The white lines most likely are due to your clients providing PDF files in which the transparency has been already flattened. This is a no-no. Again, have them use the PDF/X-4 presets in InDesign or Illustrator.
In my experience, these lines will only print or remain visible if, …
- … the printer does some kind of post processing, like opening in Photoshop and print that opened file.
- … in the post processing, the color space is converted, as it happens when someone exports a RGB-PDF or an EPUB with images in CMYK or a different RGB space.
For any purpose, as placing in InDesign or Photoshop use only PDF/X-4 with live transparency and images with their original color space, never pre-converted to CMYK.
Dear Mr. Isaacs, I respectfully salute you !
Isn't it unbelievable that this phenomena is still around ?
And that no-one else but especially you has to show up to tell users, remind users, and explain it to them once again, until you understandably come to the point you'd want to shout it out: flattening CREATES these effects ! And the lines surely will be a risk if the printing staff doesn't know how to stitch them ! (Oh, the horror.) Just like the more risky effect of getting slightly mismatched coloured areas of flattened imagery or shadows, and – the loveliest of them all – boldly pixelated texts.
E.g. this morning, I had to enforce another worried InDesign user again to neglect the advice of her printer:
"Don't use transparant elements in a design, that's dangerous."
Some of these printers just want to return to the old days of lithography...
"Convert all RGB files immediately to CMYK mode in Photoshop."
And of course, the lazy staffer even didn't supply any profile information.
"Don't use JPEG for the final quality of photos, only TIFF or EPS."
I bet they also prefer harsh clipping paths to plain transparant PSD files.
I told her to just turn to another printer, please. She didn't believe me, so that's how I ended up on this forum discussion, to supply her with some reassuring information. After 16 years of "PDF/x-1A:2001" this ghost of PDF 1.3 still haunts us...