DOI: Digital Object Identifier
Most scientific publications have a unique ID: DOI. For example, the paper "Cui et al (PLoS Biol 2009)" has DOI "10.1371/journal.pbio.1000167". DOI gives you two advantages:
1. Find the paper's webpage (usually on the publisher's site) quickly. If you have the DOI of a paper, the DOI system (http://www.doi.org/), which Peaya Paper uses, can automatically direct you to the paper's webpage where you can usually find the PDF version of the paper.
2. Find the paper's meta information more quickly. When you add a paper manually, you might find it a pain to enter all meta information such as title, authors, etc. In that case, you can simply enter the DOI of the paper, and click button "Match PubMed". Peaya Paper will automatically search PubMed and complete the entries for you.