I start a file in Illustrator with these settings:

Illustrator file settings

I save the file with these settings (no PDF compatible file):

Illustrator save settings

The AI file weighs 1.1 MB.

I place a JPG picture from a DSLR camera which weighs 4.4 MB, rescaled to A4 size. I save it and it still weighs 1.1 MB.

I save it as PDF with the standard settings, except that I uncheck "Keep Illustrator editing capabilities":

PDF save settings 1

PDF save settings 2

The PDF now weighs 33.1 MB.

I would expect the PDF file to simply embed the JPG image with coordinates and thus be only be marginally larger than the photo, or maybe a bit more for thumbnails. Instead, the PDF is more than 7 times larger than the original.

Why is the PDF so large?

  • 1
    ZIP compression is not JPEG compression. – Billy Kerr Jul 24 '20 at 16:29
  • 1
    ZIP is essentially similar compression than PNG so its like you just turned your image to a PNG. Needless to say recompressing a jpeg as a PNG has that kind of effect on you. – joojaa Jul 24 '20 at 20:04
  • PDFs also include other data.. pages, display options, accessibility, etc. – Scott Jul 24 '20 at 20:30
  • Does this answer your question? Why is PDF much larger than PNG for a vector design? – Rafael Jul 24 '20 at 22:42
  • Billy, Joojaa, and Rafael: you were right about compression. I made a mistake and thought that Illustrator only compressed if I asked it to downsample raster images. When I changed compression to JPEG instead of ZIP, I got a file size of 400 KB with JPEG of minimum quality and 2.3 MB with JPEG of maximum quality. The results for CMYK and RGB as color modes are similar: 33 MB with ZIP, 2.3 MB with maximum quality JPEG compression, and 400 KB with minimum quality JPEG compression. Does any of you want to write an answer? – miguelmorin Jul 25 '20 at 7:59

Yo do not have a JPG file anymore.

When you embed a photo inside a document, you do not have a JPG file anymore, you have a document with a photo inside. And this image is now subject to the new file's compression rules.

The abrupt change in the file size of the PDF is most likely due to two things:

You do not have an RGB file anymore. You are converting it to CMYK which has, not only more channels but compresses less efficient than an RGB file. You could even use some JPG compression but this compresses every channel separately, versus the way a standard RGB file is compressed, where a luminescence channel and two chroma channels are used.

You are not using a JPG compression, but a lossless ZIP one. But even this one would only apply on similar grounds if you maintain an RGB file.

Keep in mind that you are NOT saving a PDF, you are exporting a file to PDF. The difference is that a saving tries to keep the data as it is as possible, including some effects that are reloaded and re-rendered, while an export makes a series of processes, like converting files, resampling, merging layers, antialiasing, converting vector effects to raster, among other things, depending on the target format and the configuration.

  • I thought that Illustrator having PDF in the "Save As" menu instead of the "Export" menu meant what it says. Thanks for the tip that saving as PDF is really an export. – miguelmorin Jul 25 '20 at 8:00

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