If you want to add a PDF, PowerPoint, or Word document to an existing notebook/document in your GoodNotes library, there are three ways to do it:
1. Use "Import Above/Below"
Open the file in GoodNotes and tap on the + button in the toolbar. Then choose "Import Above/Below". GoodNotes will ask you from where you want to import a document. Import Above/Below also works in the thumbnail view. Tap on the + button in the upper left of the thumbnail view pop-up and choose the spot where you want to insert the document.
2. Import the document first and merge them
This is not the recommend way but you can still do it if you have already imported the document into your library. Here is how to merge document: https://goodnotes.zendesk.com/hc/en-us/articles/202167325-How-to-copy-and-paste-pages-from-one-document-to-another-document-
3. Use "Open In GoodNotes"
Open the document you want to add another document to in GoodNotes at the right spot. Then open the app where the file you want to import is saved (Safari, Dropbox, email...). Tap on the export button and select "Open In GoodNotes". The GoodNotes app should open and allow you to import the document above or below the open page. Alternatively, you can create an individual document from it.