Converting to CMYK upon export blows up PDF file size exponentially – huge PDFs

[Aloof]

Hey,

after a night of trying I found out that if you check the option in Publisher to convert to CMYK, my 44 page brochure gets blown up from 130MB to 300MB. The 130MB PDF converted in Adobe Acrobat to the same color space results in a 167MB file.

Files this big (300MB) are impossible to pass onwards to printers. There is something seriously wrong with the export algorithm!

The settings are as attached in the thread. Checking the "Convert image colour spaces" makes the PDF size explode.

The file document setup color setting is set to CMYK.

[Dazmondo77]

12 hours ago, Aloof said:

Files this big (300MB)

Don't you mean 3GB which also shouldn't be a problem for most print service providers via direct upload

[Gabe]

Hi @Aloof,

Can you attach the project file in question? 

[000]

167 -> 300 doesn't sound too bad, that's not MASSIVE, considering it's 375dpi and CMYK in production quality.
What kind of project is it? A book, catalogue, brochure? How many pages? How many images? How big are the images (full page?)?

As Gabe said, posting the original file will help. I'm happy to take a look and experiment with it to see, where the PDF size is coming from and what you could do to reduce it.

[thomaso]

On 11/5/2020 at 10:13 PM, Aloof said:

Checking the "Convert image colour spaces" makes the PDF size explode.

On 11/6/2020 at 12:41 PM, Gabe said:

Can you attach the project file in question? 

EDIT: sorry for the misleading (grayed-out) thoughts below. In fact if all images are set to get converted on export then their initial 24 bit are expected to turn to 32 bit, just for the additional color channel in CMYK versus RGB. – Also the embedded profiles seem to be reasonable, considering that the PDF/X-3 specification requires profiles to be embedded.

So I have no idea yet what causes the increased PDF size of ~ 100% with an APub export towards the conversion done within Acrobat (as initially mentioned by the OP).

[  In a test with RGB.tif – RGB.jpg – CMYK.jpg – RGB.jpg & blur adjustment – RGB.jpg & opacity mask on CMYK background:

Apparently with "convert images" ALL images get exported with 32 bit color depth in the PDF – whereas without checking "convert" (= default preset) plain RGB images maintain their 24 bit color depth while only the CMYK image (with differing profile) + the images with adjustments are exported as 32 bit.

Additionally, if "convert" was ticked the divergent CMYK profile appears to be embedded in the PDF – though it might be redundant since the image got converted to the PDF/X-3 output intent profile during export. Depending on the number of various profiles and their single file sizes this might increase the output file size remarkable, too, (though less relevant in documents with many images).  ]
 

On 11/6/2020 at 12:47 PM, Jens Krebs said:

167 -> 300 doesn't sound too bad, that's not MASSIVE, considering it's 375dpi and CMYK in production quality.
What kind of project is it? A book, catalogue, brochure? How many pages? How many images? How big are the images (full page?)?

Note that in the OP it is not the difference between RGB vs. CMYK export. However, even then I would expect an increase of ~25 % (for the 1 additional color channel), not of ~100 % as in the OP, or of ~ 50% as in my test with 4 images only (17 > 25 MB).

I don't understand for this OP's comparison of two CMYK exports why the "375dpi and CMYK in production quality" and the image dimensions should influence the difference in the exported file sizes.  Also I expect the media type "book, catalogue, brochure" and the number of pages to be irrelevant, not influencing the output file size any remarkable.

Edited November 7, 2020 by thomaso

[000]

19 minutes ago, thomaso said:

"375dpi and CMYK in production quality" and the image dimensions should influence the difference in the exported file sizes.

My assumption was that, with the colour space conversion unticked, the images would have been saved in their original format, maybe RGB or (if it's a bug) even Greyscale. Your explanation of everything being upscaled to 32bit definitely explains the file size.

My other argument was / is that 300 MB for a 44 page brochure is not unusual, depending on the amount and size of the images (if you have a full page image in 375dpi exporting it as RGB or CMYK and a different bitrate will definitely make a substations difference).

[thomaso]

52 minutes ago, Jens Krebs said:

My other argument was / is that 300 MB for a 44 page brochure is not unusual,

300 MB for 44 pages (= images) can be normal of course. – But doesn't both, the OP and your "167 -> 300 (...) that's not MASSIVE", concern the relative, proportional increase of ~100%, and the absolute 300 MB secondary only? Possibly a misunderstanding, I was just confused by your idea that the ~100 % increase would not be "massive", considering that ~25 % for the additional color channel could be sufficient.

[000]

On 11/7/2020 at 12:51 PM, thomaso said:

I was just confused by your idea that the ~100 % increase would not be "massive",

I stand by that -- while it is a substancial increase in size, neither is 300 MB "massive" (the term is obviously subjective) nor inexplainable for this kind of project. It's good for the thread starter if they can squeeze it back to a smaller size in Acrobat without quality loss, but I personally would not think twice about sending a file this size to any printer.

 

[Aloof]

The point is that we have the same conversion happening in Acrobat (just using color conversion, no other optimizing) clearly doesn't double the size of the file twice. I am used to file sizes from InDesign. Affinity Publisher easily doubles or triples the sizes for the same PDF-X settings. That is strange.

Combined with the slow performance of Publisher when inserting text and other bugs, I cannot consider this tool for any work, unfortunately.