This sounds like a common task, but the problem is that from the GMail App there is no way to directly transfer the file to Dropbox. The trick is to use Safari.
- First, Open the attachment, Click the “Share” button and select “Open In Safari“
- Then, within the pdf canvas in Safari, Tap once to see the “Open In” bar and from there Select “Open in Dropbox“
- Finally, You will be prompted a screen to select which Dropbox folder you want to save in, then it will proceed to upload
- There you go!