A lot of times when I re-save or optimize a pdf in Acrobat I get a warning dialog that says something to the effect of "This document contains attachments which were not included..." What is an "attachment" to a PDF? On a lot of these files I created the original and I never purposely did anything that I thought could be considered creating an attachment. And I never notice any difference before or after it supposedly strips off the attachment. What are these attachments and where do they come from?
I'm certainly never manually attaching anything. Usually they're made by printing to .ps and distilling. (That's how our prepress department requires the files be made, and I prefer to do it that way in most cases since it minimizes preflight errors.) If there are flattening issues I export directly from the original application to PDF. I don't know of a step that would automatically turn anything into an attachment. I can check the Attachments Pane next time and see what attachments it thinks are there. That might help me figure out where they're coming from.
It doesn't seem to create any problems to let Acrobat strip them from the file. I've just been wondering what this means.
It's possible that attachments are being added by Distiller. For example, if you have the "Save Adobe PDF Settings inside PDF file" and/or "Save Portable Job Ticket inside PDF File" (I think) selected in Distiller's Advanced settings, it will include those things as attachments. The former will be a .joboptions file and I'm not certain of the latter.