Did you save the signature in preview under annotation, or as a seperate PDF file.
If its a seperate PDF file then ypu can right click on it, and then you can select Open with -> Preview. Or you can Right click on it and select Get Info. Under the open with section, you can change it to Preview.
If you do not know how to save signatures in the preview app directly, here is an article on that.
https://support.apple.com/guide/preview/fill-out-and-sign-pdf-forms-prvw35725/mac