PDF documents can be imported and edited depending on how the PDF was originally exported.
The importing of PDF documents is a one-way process. You cannot overwrite the original file once it has been imported, instead saving it as an Affinity document. However, you can edit the PDF content if page objects remain editable from the original export; you can then export the file as another PDF.
If the PDF document has stored Document Properties (metadata) such as Title, Author, Subject and Keywords (tags) these will be retained on import.