How do I combine PNGs into a PDF without losing quality?
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 1210â€†Ã—â€†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?
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.
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.
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.