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I have a few hundred high-res PNGs I'd like to combine into a 200+ page PDF using only lossless compression on the images that are becoming the pages of the PDF.

But when using Adobe Acrobat's 'Combine Files into a Single PDF' feature, Acrobat always noticeably degrades the quality of the images when converting them and assembling them into a PDF, even when I have all of Acrobat's PNG->PDF conversation preferences set to use lossless compression only.

I'm on Mac OS 10.11.1 and using Adobe Acrobat XI.

ETA: Each PNG is 1210 × 1450 pixels. Yes, I'm viewing the PDF at "actual size." The document properties on the resulting PDF says the page size is 16.81 x 20.14 in.

How can I assemble high-quality PNGs into a PDF without degrading image quality?

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    what page size is it, what size are the final images on the page, and what are the pixel dimensions of the original PNG files (pixels, not ppi)
    – Yorik
    Nov 5, 2015 at 20:50
  • Are you viewing the resulting PDF at "actual size" (100%)? By default Acrobat "zooms to fit page"
    – Scott
    Nov 5, 2015 at 21:44
  • The resolution that Acrobat shows is not "infinite". At a certain threshold it just stops zooming in, even though the data is there. With Acrobat Pro you can use the Object Inspector to check the resolution of your images.
    – Jongware
    Nov 5, 2015 at 23:21
  • Each PNG is 1210 × 1450 pixels. Yes, I'm viewing the PDF at "actual size." The document properties on the resulting PDF says the page size is 16.81 x 20.14 in.
    – lukeprog
    Nov 6, 2015 at 4:49

1 Answer 1

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The page size (16.81 x 20.14 in.) is exactly the number of pixels at 72 ppi. There's nothing abnormal there.

The question is: What page size do you expect?

Your PNG are low resolution and Adobe Acrobat simply take what you have and doesn't modify them. If you expect a different page size or would prefer a higher resolution and smaller size, you'll probably need to resave your PNG with a higher resolution or maybe have a look at the preflight functionality of Acrobat Pro to modify the resolution quickly within the PDF.

To come back on your issue of Acrobat lowering the quality of your PNG: One thing you might want to verify is the resolution of your Acrobat preferences. If you use the default setting, it might show your image bigger when viewing at 100%. If you use 72ppi then it should show you the images as they truly are.

It's possible this is what makes the images look lower resolution. So it's worth trying a different setting that fits your screen.

Adobe Acrobat Pro resolution

You can also have a look at the PNG to PDF conversion preferences; by default they seem to be at medium quality. Maybe that's what you're referring to in your question and you already verified this.

convert PNG to PDF

If you aren't sure if your Acrobat is doing a good job, simply open a few of these PDF in Photoshop at their native resolution. If the image is the same and you're happy with the results, then the issue was probably only the resolution preview in Acrobat.

Acrobat doesn't always show images, texts and vector in a perfect way, and there's a few settings in your preferences that don't affect the file but still display them differently than in a software like Photoshop.

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  • Thanks, @go-meek. Yes, my Convert to PDF -> PNG settings are all set to lossless formats, as you show. I changed Page Display -> Custom resolution to 72ppi, but the PDF version still shows the quality degraded. I also tried turning off Acrobat's font smoothing, but that didn't help. To illustrate, here is a snippet from one of the PNGs, displayed in Preview at 'Actual Size,' and here is the same section from the resulting PDF, displayed in Acrobat at 'Actual Size'.
    – lukeprog
    Nov 7, 2015 at 6:23
  • @lukeprog The smooth text should only work on real text in vector, not png. It does look like there's compression to JPG actually. Are you sure you're not using any JPG compression? Even if you use lossless on monochrome, your png might not be monochrome; monochromes are usually lineart 1-8bits, bitmap modes, etc. The other black and white images are considered grayscale and if you're in RGB it's going to be color. I didn't have time to make tests but if this was my projects I'd try to 1) resize my images with a batch script to higher resolution & 2) try the other compression mode
    – go-junta
    Nov 8, 2015 at 7:53
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    At your suggestion, I tried another compression mode (ZIP instead of JPEG2000 lossless) and that worked! My eyes can't see any difference between the quality of the PNGs and the quality of the PDF, now. Apparently "JPEG2000 lossless" is not, in fact, lossless! Thanks for your help with this.
    – lukeprog
    Nov 9, 2015 at 21:47
  • Woah, so now when I did text-recognition on the file and saved it (Cmd+S), the filesize dropped from 222MB to 167MB, and the jpeg artifacting appeared again! Any idea what setting is causing it to lose quality when I save after doing text-recognition? I checked all the Acrobat Preferences and I'm not seeing any reason it would do that.
    – lukeprog
    Nov 9, 2015 at 22:42
  • @lukeprog Maybe you have a mix of your previous png and vectors. Hard to tell. But you can try optimizing your pdf, files aely get above 50mb: graphicdesign.stackexchange.com/questions/49315/…
    – go-junta
    Nov 10, 2015 at 0:54

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