Hi Jeremy,
Thank you for contacting Syncfusion support.
Currently there is no support to add the hyperlink to the selected text in the PDF document using the Syncfusion PDF Viewer. However, you can add the hyperlinks for newly added texts using the Syncfusion PDF Base Library. We have shared the UG documentation link for your reference.
Kindly try this and revert us, if you have any concerns.
Regards,
Dhivya.