Printer margins

[David23930428qirn]

Hello,

I'm trying to create a .pdf that prints double sided, which will be folded. The images on both sides need to align correctly. I can print them single sided to the printable margin for my printer which is 1/4", but when I combine the .pdfs and print duplex the printable margin increases to 3/4". I can't find any difference in the document setup for the pdfs. Why is that happening, I can't figure it out. Files originate in AI but I can't see how that's relevant.