To save documents to Dropbox (or any other application that uses an iOS extension and supports PDF files) you must have the application installed. First, download and install Dropbox from the App Store. Launch Dropbox and sign in to your account (or create a new account.) Next, switch back to ModMath and tap the share icon in the library or document view. In the bottom row of the sharing window tap the “More” icon. In the displayed list find the option that says “Save to Dropbox” and toggle the switch to the “on” position. The Dropbox icon will now be displayed in the sharing window, tap the icon to save your document to Dropbox.