Optimize PDF corrupts some images but not others

Question

Hello! I'd like to optimize some PDF courses and the Acrobat DC optimizer works well for around 90% of the graphics but some graphics are so corrupted that I won't be able to use the optimized file.

Here's an example of corruption - graphic on left is original, graphic on right is optimized:

For comparison, here's a similar graphic on another slide: original on left, optimized on right looks great:

Any ideas?

I'm using Acrobat Pro DC Version 2021.007.20099.

I tried Tools > Optimize PDF and Save As with Reduce Filesize chosen, same result.

thanks!

Lori

lorib52691051 · Accepted Answer

Hello all, here's the resolution for this issue:

The Acrobat optimizer (Acrobat Pro DC > Tools > Optimize PDF) worked well on all of my pages except for pages that had a .jpg image anywhere on the page. The reason those pages looked junky was because I was using the default optimizer settings which include a default JPEG Compression Quality setting of Medium. Apparently, if there are any .jpg images on the page, the optimizer attempts to preserve the .jpg quality by processing the entire page as a .jpg. This does not work out well if you have any elements on the page that are not .jpg.

My solution was to go to the Advanced Optmization screen and choose Maximum Quality for JPG Quality Compression. This did result in an increased optimized file size but at least the images looked reasonably ok.

Lori

gary_sc · Answer

Hi Lori,

What you're seeing in the first example is a classic case of JPG degradation. The JPG file format is (what is called) a Lossy format, in that when it compresses, it loses data. Since it can't just delete pixels willy nilly, it takes the pixels/information from a near adjacent pixel. When you have a photo of a tree, you'd never see this. But if the tree is adjacent to a clear blue sky, that's what you see.

I "appears" that the blue and gray in that first photo are jpg images but the rest of it very likely is either a PNG format or a GIF format. Both of those are not lossy and display limited color images excellently. However, I've never heard of, nor have seen of a single image that is both. So I'm really not sure what's taking place there without personally examining the document.

Meanwhile, when you do an Optomization of a PDF, there's only so much it can do with text to make the document smaller. So it must rely upon any images to optomize the document. If it's important to optomize everything, about the only thing I can think of is to take all of the images and place them in Photoshop (or somethign similar) and save them as either GIF or PNG images. Low color range images like you show can be cut in storage size significantly by such conversion.

If that's not an option, one other thing you could do is to bring the document into the Organize Pages, see which images are damaged by the Optomization, and replace those pages with non-Optomized pages/images. This way the ones that can't be affectively optomized will still appear just fine while those that can be optomized will carry the brunt of the size compression.

Yeah, lots of data that is probably not very relevant to you but that's the best explanation I can think of.

Good luck!

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