The only way I can think to achieve this is to embed a pdf in the page and add the signature plugin under the document and it can be signed directly in adalo. There would be no way to download the pdf or signature but it is stored in the Adalo database.
For more info on how to embed a pdf, see the following thread Unable to embed a PDF via the web view component - #2 by zachbharris
The only other way I can think of is design the interactive pdf which includes fillable fields and a signature field outside of Adalo and then just use Adalo to send and receive the blank and signed documents.