I've come across a weird behavior regarding the output file size when exporting/saving files from Illustrator CC.

I have an image with several bitmaps (i.e. raster images/photographs) where I added some annotations, markings, paths, etc. The photograph occupies the whole artboard (which is set to my desired output size), vector features are therefor only on top of the photographs.

Now when I export this as JPEG the file turns out to be around 10 MB large. However, if I save the image as PDF and select JPEG compression, the image only turns out to be 3.5 MB large.

How can that be? Quality should be the same, detail is set to 300 dpi in both cases. I thought if the PDF embeds the JPEG, it should be more or less the same size, and adding PDF container overhead, the PDF should rather be bigger than the JPEG. How can one explain the ~60% smaller file size?

  • Well, most probably the mages themselves aren't actually as big as you think. Any image can occupy any size in a illustrator file and the PDF options save images at whatever DPI they actually occupy or 300 if denser than 300PPI. – joojaa Mar 8 '19 at 18:13
  • But like I stated I specifically turned on resampling to exactly 300 dpi even in PDF export. Also, the images definitely have a higher resolution than 300 dpi. – TJJ Mar 9 '19 at 0:00
  • Yes but it does not resamples up only down. So if you have any mistake in your thinking that explains it. Remember theres no DPI in illustrator so if you scale a image its nolonger as dense as you think. – joojaa Mar 9 '19 at 7:29
  • I thought Illustrator doesn't do any resampling? So, if I load a picture, it might get scaled for display, but the data doesn't change. Especially, if I am just linking pictures instead of embedding. Resampling is only done if exporting to raster file formats. Also, I don't believe it doesn't upsample. Otherwise, how do the file properties of exported files show the possibly higher DPI? – TJJ Mar 10 '19 at 15:30
  • @TTJ no illustrator does not resample, PDF output does! However, if you rescale your effective DPI does change! Since DPI is dependent on size. Data des not change, but DPI is not a inherent property of a picture, it changes. So if the image is larger in size than originally imported as then the DPI is lower than you think. So there are 3 sources for this 1) your dpi isnt what you think it is. (this is hardly surprising about 80% of people who think they understand DPI/PPI/LPI do not) 2) your image contains a lot of empty space 3) something extra is embedded in image. – joojaa Mar 10 '19 at 19:15

When you push this out as a .pdf, all your annotations overlaid over the JPG image are staying vector data, whereas when you push it out as a single unified .jpg, all those annotations are being rasterized on the fly, and given that .jpg compression is nearest-neighbor, that adds a lot of minor areas of difference which have to be encoded and cannot be pushed as 'nearly the same as neighbor' into lossy similarity - so honestly it's not at all surprising - especially if you have a fair amount of text in your annotations - text represents a lot of very tiny areas of difference - which is super-hard to compress - but is quite small as vector data or even retained text data.

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    Thank you for your input. I had thought about this myself, so I just removed all the annotations. It makes almost no difference to the file size of the exported JPEG (just a few KB), while the PDF still is vastly smaller. It seems the internal JPEG conversion for embedding in PDF works differently. I am using CMYK for both exports. Any more ideas? – TJJ Mar 7 '19 at 23:31
  • Wow - that is fascinating - I've not a clue then - I think we should see if we can get @joojaa to weigh in on this thread - he has a programming / dev background and knows far mroe about the back-end stuff than I do - maybe he has a suggestion? – GerardFalla Mar 7 '19 at 23:40
  • I'll try to setup a set of sample pictures and attach them. – TJJ Mar 10 '19 at 15:31
  • see the attached file in the comment to my original question, please. – TJJ Mar 16 '19 at 16:17

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