Since this never has been answered properly I will share an solution which probably exactly does what you are looking for. The question is not 100% clear but if I understood you want to convert a .HEIC image to PDF and afterwards annotate the PDF. For both you can use Converter App. The conversion from HEIC to PDF can be done here. After completing this process click "Files & More" and enter "PDF Editor" in the search box to access the PDF Editor. You can submit the PDF there and start annotating it.
Assignment
Document converter that supports HEIC images.
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