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I thought "press quality" would have the most information embedded, so I would have expected those files to be bigger.

Thanks.

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I just created a test file with a only simple 6 letter word set in Myriad regular. And when diffing the two PDFs, the color profile embedding area of the High Quality Print PDF was the only place there was significantly more information stored, about 500kb more in fact. in a 650kb file. There was still a 4kb discrepancy when matching the color profiles settings from Press and High Quality Print, so there are likely some differences in how the fonts are embedded as well.

The bottom line seems to be that Press quality assumes that you will be printing on a 4 color press (CMYK Colorspace), and modifies the color information in a potentially lossy way.

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It'd be great if you could add results of a test case with a very high res embedded image as well, to determine how it's compressed/resampled. –  e100 Mar 15 '11 at 8:54
    
I would also be curious to see how InDesign creates a similar PDF as compared to Illustrator. –  Philip Regan Mar 15 '11 at 9:49
    
When comparing settings using the "edit" dialog from the pdf printer settings dialog, I see that "press" converts color to cmyk (high quality leaves unchanged), and "press" also stores OPI comments and PDF comments, which high quality does not. All other settings appear to be identical. –  horatio Mar 15 '11 at 14:24
    
@Philip: A PDF is a PDF is a PDF except when it's an "Illustrator PDF" or a "Photoshop PDF". In those cases, a copy of the entire native document is saved in a private area of the PDF file, invisible to other applications, but used by Illustrator or Photoshop when the PDF is opened in the application that created it. Conversely (and perhaps paradoxically), when you save an IA file and your preferences are set to "PDF Compatible," Illustrator saves a PDF with the IA file. A non-native preset such as PDF/X is the same, regardless of the app that created it. –  Alan Gilbertson Jun 18 '11 at 0:17
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