Acrobat "reduce file size"

[michaelf42288906]

I teach college level architectural studies; for years, I've required students to reduce the size of files to control upload times. But people ask the same question every year, and I have no idea of the answer - when a file is "reduced" what's removed? And if what is removed is unnecessary, why does it stay there in the first place?

I know it's really basic, but I'd very much like to be able to give an "official" response!!!

[Karl Heinz Kremer · Accepted solution]

Reducing the file size does a number of things, some without actually changing the content, and other that will change the contents of your file: There are ways to save "stuff' in a PDF file as compressed data. This will not change the contents, but will make the file smaller. Some PDF generators don't take advantage of these mechanisms, so Acrobat can step in and do a better job.

Your document may not use all glpyh (the drawings of characters you see) of a font that is embedded, so Acrobat will "subset embed" that font, and only embed those glyphs that are actualy present. This will also not change the look of your document. 

Some applications store application level data (e.g. InDesign or Illustrator) in the PDF file, that stuff is only needed when you try to edit in e.g. Illustrator again, otherwise it's just extra data that is not required ot view or print the PDF file. 

When you have images in your file, Acrobat will try to downsample them to a level that is still acceptable for viewing the files, and may use a more effective compression algorithm for images. Both downsampling and a lossy compression algorithm will reduce file size dramatically, but this is where the end result will be different than what you started with: The image quality suffers. You may not notice that when you just look at the file on a computer screen, but when you print to a high resolution printer, there is a good chance that you will notice. 

There are probably other things that Acrobat does, to make the file smaller. If you want more control over what gets done, and you do have Adobe Acrobat Pro, there is also a save as optimized PDF, and here you have full control over what done to the file. 

[michaelf42288906]

Thank you so much! I can’t say it’s 100% clear, but it’s good information and I’m going to save the text so I can pass it on when people ask again!

Michael