help
You can't natively (is that a word?) save multiple drawings as 1 PDF file.
However, as Markku Lehtola has pointed out, there are utilities that will enable you to combine PDF files once you have saved out all of your individual drawings as separate PDF files.
There are other options as well.
Inside #TASK (TASK), it has a utility to merge drawings, which you could then save out as a single PDF file.
PDF Task – Combining files