Hi,
Yes, it is possible to opene the existing empty PDF document by using the PdfLoadedDocument class.
Please refer the below sample to open a blank pdf file insert text from a text box and then save into the server.
http://websamples.syncfusion.com/samples/PDF.Web/6.4.0.14/F78516_1/main.htmPlease try this and let us know if this helps you.
Best Regards,
Bhuvana