Limitation on collecting PDF responses with Acrobat

At the pricing site (https://www.acrobat.com/formscentral/en/landing/axifcpricing.html?promoid=KEUSK&accntlvl=f r) I see that collecting PDF responses is limited to 500 people, while collecting respones with FormsCentral is allowed for 5,000 per form. What is the difference in these two methods of collecting responses?