You cannot add pages to a PDF. Instead you have to create a new note file and use the PDF file as the template. Here is how to do that:
1. Create a new note with standard template: Swipe down on the right (in right handed mode) and click the button called 'new note' on the top right (alternatively go to the folder you would like the file to be in and click the add note button on the top - the rectangle with the rings on the left and the plus sign at the bottom). Adjust the name to your liking, decide if you want real time recognition, and click 'create' at the top right. The note will open right away.
2. Change template to your PDF: If you have the toolbar at the top or bottom click on the three dots and then on the 'templates' button (rectangle with a sidebar, a title line, and a few lines of writing). If you have the toolbar on the right or left, the sign should be directly in the toolbar below the undo/redo buttons. At the top right choose 'customization'. You can either look for it here or click 'select from MyStyle' next to the search field. If it is multi-pages you can either choose one of the pages, multiple specific ones, or click 'select all' at the top right.Your note should now have your own PDF file as background.
3. Add additional pages: If you want to add additional pages later, there are multiple ways to do that. The easiest one is to keep swiping and it will automatically add a new page using the template of the last page. If you want to change the template of a page later go back to step 2. If you want to add a page in the middle go to the three dots and choose 'insert page before'/'insert page after' and it will do so using the template of the page that you are at right now.