Where are docs located

[flongest]

Hi. I've been a PC user for years and recently got an iPad. I'm pretty lost trying to perform very basic tasks here. I installed Dropbox, which works simply on my PC, but on it requires me to be able to save files in the Dropbox folder and I can not figure out how to find my document folders on the iPad let alone save a file to them from Numbers. Consequently, nothing I edit or create on the iPad is saved in Dropbox.
How do I save a document to a specific location like the Dropbox folder?