Hi,
Thank you for your interest in Syncfusion products.
You can load an existing PDF document (using PdfLoadedDocument) and fill the fields by identifying them using the filed name or index. Please go through our online sample in the link below that explains the same:
http://samples.syncfusion.com/ASPNET/7.1.0.30/web/pdf.web/samples/3.5/Modify%20Documents/Form%20Filling/VB/default.aspxAlso, please go through our samples under the Graphics category that explains how to draw graphics using Essential PDF:
http://samples.syncfusion.com/ASPNET/7.1.0.30/web/pdf.web/samples/3.5/Default.aspxPlease try this and let me know if you have any questions.
Regards,
Geetha