More features in Pro version:
tap to search an author
tap to search the journal
related citations

Post a comment for an article to public
Write a note to yourself
View public comments for an article

search by PubMed ID, just enter the id in the search box

To migrate your saved articles into the pro version:
install the pro version
start the free version
press menu on the initial screen of the free version
then "Upgrade"

Screenshots: