Hi Damien,
Thank you for your interest in Syncfusion products.
Yes. It is possible to open an existing pdf file, stamp it with an image/text and then save the file again.
We have created a sample to perform this and attached in the below link.
Please verify the sample and let us know if this helps.
Regards,
Meikanda Nayanar.I
Sample_e8a39795.zip