Through trial and error and a bit of thought I've found the following to work...
First you must understand that when the email attachment of your pdf is formed, it is formed as if you were opening another copy of the form and then just adding the data you've entered. So the "hidden" presence is not carried over because the original property boxes for the object specify "visible"
To get it to do so, instead of making an area hidden by clicking a button, make the button set the value of a hidden number field (to 2 for example). (before doing this... place a number field on your form design and then set its presence to hidden. )
Then in the layout: ready event of the object add your code, for example
if (yourhiddentnumericalfield.rawValue == 2)
yourobject.presence = "hidden";
look up process events in the designer help files to see why this works, i.e. how the pdf is actually rendered.
don't forget to reset the value of the hidden number field after the email event if you want to continue to use the document (not the emailed one)
I may not be explaining this as clearly as I could, and there may be a better way to do so, but this is working for me and this is how i interpret the process . Comments welcome