PDF size from PNGs very small

Question

I created a 25 page pdf with acrobat from 25 png files approx 65mb each. They are 11x17 @ 300dpi so quite large. When combining files I made sure to choose the large format for print but pdf size is only 15mb - only approx 100th the size of the 25 files. On screen the pdf looks ok, but I'm just wondering how this is possible without having lost tons of data? Are the pdf pages truly the same quality as the original png files? If not, how can I create a pdf with full resolution images?

Answer 1

If your original png files are very highly compressible - mostly a single flat colour, for example - and you originally saved them without using compression, then Acrobat might choose to compress them without asking you. PNG compression is lossless, so it would probably not bother asking, just do it. then the final product would be smaller, yet would be pixel-for-pixel the same as your original.

Answer 2

Acrobat probably saved the images within the pdf as compressed jpegs.

I just took a jpg image from a camera, a landscape shot, which came out of the camera as an 8Mbyte jpeg. I opened that in photoshop and saved it out as an uncompressed png file, which was 48Mbytes. (Compressed png would have been 21Mbytes.) I opened that in Acrobat Pro to make a pdf, and saved the resulting document as a pdf. Acrobat asked me no questions, but the total resulting pdf was 1.2Mbytes, so it must have compressed the file a great deal. The picture still looked OK.

For Adobe CC Acrobat Pro on a Mac, I can choose Acrobat - Preferences - Convert to PDF - PNG - Edit Settings - color and it offers a variety of lossy and lossless conversions. The above example used JPEG (Quality : Medium).