App features and plagiarism

Hello world,

I am currently trying to design a mobile device app for academic use, with a few PDF handling features. Unfortunately, some handy features I would like to include have already been implemented in other – expensive – apps. Those features are not "generic", such as playing an audio file, but very specific and unique, such as app layout ideas, or unique PDF handling features.

Can I legally build those features into my app, coding them from the ground up, and eventually making the code open-source ?