IPhone : how can i import files from Dropbox to my app? or vice versa?

I managed to link my iOS app with Dropbox using Swift. I want to import file From dropbox and save it in my app directory. and export files in my app to Dropbox.

Like, i forgot the name, but i remember there is an pdf reader app that imports pdf file from Dropbox.

I saw there are APIs in Dropbox, but i found it very hard to understand. is there any sample codes to do this? It doesn't have to be Swift obj-c will do the work.

Thanks for any help.