Save an image (PNG) with annotations as a PDF
You can always use File > "Export as PDF…" or even File > Print… > click PDF options in lower-left corner as a way to convert anything open in Preview to PDF. However, any annotations made to an image format such as PNG without converting will be flattened/rasterized when saved or converted to PDF and can't be edited. You will need to first convert the image to PDF  then annotate in order to be able to edit annotations later.