1. Import your PDF
There are a few ways to do this.
From the New Book screen (click on the button in the top right)
From the Books icon on the bookshelf:
Either way, you'll be prompted to upload a file from your computer or from Google Drive. If your PDF has multiple pages, you can select which pages you want to upload - a new page will be created for each one.
You'll then be prompted to choose a book shape for your imported PDFs. Any size will work, but it would be best to try and match the format of your PDF.
The other way to import PDFs is from inside a book. Click the + button then More > Files.
If you do it this way, once you've selected the file to upload, you'll be giving the choice of either importing or embedding the PDF. Choose the Add pages from your PDF option.
You'll choose which pages you want to upload (if the PDF has multiple pages) but you won't be asked which format book you want, as you'll already have chosen that.
2. Editing your PDF
The next step is to annotate your PDF in Book Creator. You can use all of the tools from the + button menu: text, images, audio, video, shapes etc. to layer on top of your PDF.
The PDF you import will actually be converted into an image when you upload it, which gives you more options. You could crop the PDF by right-clicking and choosing Crop from the menu.
Or you can use the drawing tools built into Book Creator. Just select the PDF then click on the Pencil icon in the top right.
Once you've done that, you will open up the canvas to edit the PDF (if you've used the Pen tool in Book Creator this will be familiar to you).
From here you can:
Use a pen to write, draw or highlight on the PDF.
Use the fill tool to color in items on the PDF.
Use the eraser to rub out portions of the PDF.
Click Done when finished. You can always make changes by opening up the PDF again.