Buying iCloud storage will have no impact on this. You need to make sure you have sufficent storage on the device to download the file. Deleting App, unless they happen to contain large amounts of data, would also not likley resolve it. Typically Media files, music, movies, podscasts) will take up more room than anything else.
To check your curernt usage: Settings app > General Usage will give you an indication of what is taking up the most space.