There is really no such thing as a high resolution PDF versus a low resolution PDF. What you are really doing is downsampling the images within the PDF file. Any text or vector content is fully scalable and has no inherent resolution!
If you are framing images, you should be using vector-based lines or shapes instead of incorporating such “line frames” into the images themselves. As part of the images, downsampling could in fact corrupt the apparent thickness of these lines, especially if “lossy” compression such as JPEG is used.
The Enhance thin lines and smooth line art are totally irrelevant to raster images such as your photographic images with embedded frames in raster format.
Bottom line is to change your workflow to frame your images with vector-based shapes and don't incorporate same into the image itself.
Thanks for your answer but you are assuming I'm doing things incorrectly, which is not the case.
All my line artwork is vector based (yes, even the frames around images), and when I say high resolution and low resolution is partly correct because when you are reducing the file size of a pdf, images need to be compressed drastically to enable the pdf to be much smaller than a PDF that has no compression at all. Therefore, this is reflected in the file size and quality of the PDF.
I am preparing these PDFs to be viewed digitally both on desktop and mobile, as they will be part of a digital campaign. I would expect that when a pdf is compressed, there will be some loss of image quality but I don't expect the pdf to change the appearance of any vector artwork, and this is my problem.
I have googled this many times and there does seem to be an issue for many people. The appearance of line artwork changes once you create a compressed pdf, and I was wondering if Adobe has an answer for this as I couldn't find any. It did look like if it was an acrobat bug.
Any other answers out there?
Contrary to what you may have been told and/or believe, there is absolutely no change in display or print quality in a PDF file for vector or text content when you do downsampling of the images! Vector is vector and is not measured in pixels which is resolution dependent. And FWIW, at Adobe we are not aware of any issues associated with downsampling images causing issues with vector and text content.
OK, assuming that your frames are indeed vector, one other thing to look at is ordering. If the image is above the frame, the boundary of the frame and the image may be the issue. Try putting the frame above the image and see if that helps. Also, make sure the frame is opaque.
If that isn't the case, we would appreciate it if you could post PDF file samples for us, one before downsampling and one after downsampling and we'll try to see what's going on.
Thanks for your help.
I've probably confused the matter here by saying high-res and low-res pdfs because I am aware/know what you are saying, so let me give you more information/background and screenshots of the problem I'm having when compressing the pdf.
This is precisely the core of my problem. As far as I am concerned, there should be no change in any vector based artwork or in any text within a pdf, even if the pdf is compressed. Yet, the appearance of my pdf is changing when I export it as a reduced size PDF.
Here are details onhow I'm preparing my artwork:
1- I am preparing artwork in photoshop CC. It's an A4 - 300 dpi file with a combination of images, text and vector based artwork.
2- I'm saving a copy of the original photoshop artwork as a photoshop PDF document.
3- When I open this artwork in Acrobat Pro DC, it all looks exactly as my original photoshop artwork. So, everything is fine there.
4- Within Acrobat, I go to: "save as other > reduced size pdf. Once the reduced version of the PDF is exported, all line artwork appear in random thicknesses and some even disappear. This is the problem I'm having which I'm hoping you can help me understand why this happens.
So here is an example. Sorry, I cannot show you the entire document for copyright issues, but this summarises what happens throughout the document.
I need to highlight here that these lines are vector based.
Thanks for providing the technical details. When you Save as PDF from Photoshop, vector or text objects drawn in Photoshop may or may not end up as vector or text in the PDF.
(1) If you flatten the layers before saving the PDF, vector and text is rasterized. There may be some other situations that such rasterization would also occur implicitly.
(2) If the layer with the frames is beneath the image or at the edge of the image, image downsampling may overlap the vector frame.
If either of these situations are true, I can readily imagine the symptoms you describe occurring.
Again, if you can post a sample of such a PDF file or send a copy to me at Adobe <email@example.com>, I'll gladly take a look at it and give you a definitive answer.
Thank you, what you are saying makes sense. My only question is why the original pdf export is ok, but when I save a reduced size pdf this happens. If the layer rasterisation happens then it would probably show in the original pdf too, but it doesn't.
Also, there is no change at all to the text. It still displays as crisp as the original.
I have sent you a couple of pdfs by email.
Many thanks again.
personally I'd expect the lines the be rasterised even if the text isn't. Why are you using Photoshop for such unsuitable work?