It seems that when you are copying static text from the PDF and pasting it into the text field the font size is being increased by something like 20%. Is that the problem you are facing. I must say I haven't come across it as the main copy/paste I've had to deal with is from MS Word, which does seem to work. Even if you past from the PDF to a MS Word document it gets enlarged as well.
So, sorry to say after a couple of weeks I can't help.
The text that i copy is an example text, i try all method to copy/past the text "from text to textfield" or "from textfield to textfield" with "Rich Text" and the results is the same. This is the link to my sample: ExpireBox | NewForm1.xdp
I you set the text field to a Field Format (on the Field tab of the Object Palette) to rich text, then the value pasted will retain the source font size. Otherwise it will change to whatever has been set for that field.