To move a PDF file from dropbox first open the file in the dropbox app. Next tap the 'more' icon in the top right (three circles).
Next tap Export, and then in the export menu tap Open in... at the bottom.
You should now be able to find the Copy to Buildcloud icon in the menu, tap this and the file will be moved over to Buildcloud.
The app will try and find a similar file, in case your file is a new version of an existing drawing, but if it doesn't find a match you can tell the app what type of file it is and which project the file should go in.