Will Mod Podge work with PLA prints?
From what I've read about Mod Podge, it is an adhesive with a vinyl acetate base. As such it is similar to both PVA (used for wash-away support) and ordinary white glue. One of the more common references to the product refers to it not being water proof, although the outdoor version of the product presents as being water-resistant.

As part of the research for this answer, I found references to overly-thick coats becoming milky. This follows a reference to applying thin multiple coats and allowing proper cure time between coats. That proper cure time is listed as 28 days. 

Another set of posts suggest to cure the MP more rapidly than 28 days, one can heat the item in an oven to 175°F (80°C) which should not cause the PLA to melt, but may allow for sagging of unsupported parts. Testing is recommended.

MP is an adhesive, is known to stick to non-porous surfaces and would be no more harmful than glue stick for a PLA (or ABS) model.

Durability is uncertain. If you intend to paint the item after sealing/smoothing, you'll gain durability and water resistance.