This seems to be the best dedicated app for PubMed. It has a powerful search syntax and clearly shows which articles are FullText or FullText Free (assuming PubMed hasnt mislabeled them). You can either save articles to the built-in library or save out the pdfs to the pdf reader of your choice. I cant think of any way this app could be improved!
YesNoCancel about PubMed On Tap, v2.6.1