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I have several Illustrator files that have multiple embedded bitmap images. Some are photos and some are detailed line art.

When exporting to PDF, it is possible to control downsampling and compression for images, but the settings apply to all embedded images equally.

How can I compress only the photos as JPEG and leave the line art images untouched (or compress them less)? Can compression and downsampling be controlled separately, image by image?

The line art images contain some colour (red, black and grey lines), thus I cannot just convert them to monochrome (or even grayscale) and disable downsampling for these specific images types in the PDF export settings.

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  • I would actually trace any line art if it's all in Illustrator. The vector tracing will be much, much smaller in terms of kilobytes. Then you could merely alter compression for the remaining raster images.
    – Scott
    Commented Mar 6, 2019 at 16:01
  • @Scott Tracing them is not an option. It's complicated technical drawings ... They're fine as bitmaps and not huge (they compress very well e.g. as PNG due to the flat colours / white background). What I want is to compress all the other images (photos) while leaving these alone.
    – Szabolcs
    Commented Mar 6, 2019 at 19:25
  • The use of ZIP compression would reduce the file size without altering the quality of images
    – spike_66
    Commented Mar 7, 2019 at 8:08
  • @spike_66 ZIP compression is much less efficient than JPEG on photos. Using only that will keep the PDF too large.
    – Szabolcs
    Commented Mar 7, 2019 at 13:06
  • @Scott Maybe one solution is to not embed the bitmap images into the Illustrator file, just link them. Then compress the separate files as needed (JPEG/PNG/etc.) Unfortunately, currently I have all of them embedded. If there a quick automatic way to convert embedded ones into linked files?
    – Szabolcs
    Commented Mar 7, 2019 at 13:07

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For my situation, I need to reduce and export the Adobe Illustrator project file from 515MB to a PDF under 25MB. Image quality is important because the document is a creative portfolio with vector graphics, text, line art, and linked images (e.g., digital assets, photography). Utilizing the custom settings listed below, I've managed to export the PDF to 21MB.

Export Setting Configuration
Optimize For Fast Web View [Checked]
Color Bitmap Image Bicubic, 750-1125PPI, Auto [JPG], Quality [Max]
Grayscale Bitmap Image Bicubic, 750-1125PPI, Auto [JPG], Quality [Max]
Monochrome Bitmap Image Bicubic, 1500-2250PPI, CCITT [Group 4]
Compress Text/Line Art [Unchecked]

With that said, one of the featured projects included in the document is a user interface that sources relatively small profile images being overly compressed. I would like to omit from the original, document-wide export compression to prevent pixelation when avatars appear next to interface components and recipients zoom into view. I'm currently exploring three options:

  • File Management: Ensure linked images are imported with the compression already applied to manage the overall file size. When exporting the PDF, one can (theoretically) do so without adding another layer of compression.

  • Adobe Acrobat: Import the compressed PDF into Acrobat. Find the compressed image and replace with the original file. Saving the work will increase the file size depending on the number of compressed images being modified.

  • Presentation Style: If the layout is set up as a presentation, one can export the document in batches (compressed) or single files (non-compressed). From there, assemble and organize the final version in Acrobat for a balanced PDF file size.

I'm considering the second option as the appropriate solution for my case given the amount of work to export and replace assets when I only need to modify a select number of images. Option three exists. But I value consistency, and navigating between slides from compressed to non-compressed pages will be drastic. Either way, I think a feature request with the ability to manage compression settings at the linked or embedded image level via 'Properties' or 'Links' panel might be worth exploring.

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