Sorry, I found discussions that address this. Many discussions are about this subject, but do not actually answer the problem, so I'll summarize the various guidance from the helpful discussions that I found here:
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Basic Setup (borrowed from someone else, but I can't remember who)
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First, you have to save the documend as a dynamic PDF, not a static PDF. That's important. Then, when you set up your document as described below, the field will expand across pages ONLY after you tab out of the entry field.
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Form Design (borrowed from Steven Dayton)
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On my first page I have a sub form and within that sub form I have another sub form that contains an expandable text field. The specs for the parent sub form is as follows:
Object tab > Subform tab > Type: = Flow Content.
Object tab > Subform tab > Allow Page Breaks within Content is checked.
Object tab > Pagination tab > After: = Continue Filling Parent.
Object tab > Binding tab > Repeat Subform for Each Data Item is checked and Min Count is set to 1.
Layout tab > Auto-fit for the Height is checked.
The specs for the subform inside the parent sub form is as follows:
Object tab > Subform tab > Type: = Position Content.
Object tab > Subform tab > Allow Page Breaks within Content is checked.
Object tab > Pagination tab > After: = Continue Filling Parent.
Object tab > Binding tab > Repeat Subform for Each Data Item is checked and Min Count is set to 1.
Layout tab > Auto-fit for the Height is checked.
The specs for the expandable text field is as follows:
Object tab > Field tab > Allow Multiple Lines and Allow Plain Text Only check boxes are both checked.
Layout tab > Expand to fit check box for the Height is checked only.
I have recently posted a question about how to get an overflow leader to show up only on overflow pages. Don't have the answer yet, but you can search for it later if this is something that you might want also (as I imagine most people would).
