You can bring a PDF into Sketch, but you must bring it in from the Creative Cloud Files folder on your desktop. So drag it there first. After you do that, you can place it in a Sketch canvas as you would any other Photo Layer artwork (it can take quite bit longer than placing a photo so make sure you have a good/fast wifi connection):
1. With a Sketch artboard open, tap on the Graphic icon (landscape photo) in the toolbar.
2. That will open a pop-up menu with the ability to select an image from: On my iPad, Take a Photo, My CC Files, From Market, From [Adobe] Stock, Paste Image. Choose My CC Files.
3. Once the image is on the canvas, tap the Layer icon (three squares) and choose the bottom layer (the bottom layer and top layer are photo layers and the middle layer is a drawing layer) and set the opacity using the slider.
4. Tap done and you'll automatically be taken to the drawing layer.
Hope that helps.