You can send PDF files over to the Buildcloud app by using the share functionality on your iPhone.
First open mail, then find and open your PDF. In the bottom left tap the share icon, it looks like this:
This will bring up the share menu, scroll over to the right and you will find Copy to Buildcloud. Tap this and your file will be sent over to the app.
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.
The more drawings you add to your project, the easier it gets to add a new version, as the app will find the original file and you just tap one button to confirm the drawing is a new version and upload it to the right project.