Thank you for using Syncfusion Products.
PDF document does not have DPI, since it is a vector format.
However, we can get the height and width of the PDF document by using the PageSettings of PdfDocument. Please find the below code snippet to get the height and the width of the PDF document using the PageSettings of PdfDocument.
PdfLoadedDocument ldoc = new PdfLoadedDocument("../../Data/Barcode.pdf");
PdfDocument document = new PdfDocument();
PdfPageBase page = document.ImportPageRange(ldoc, 0, ldoc.Pages.Count - 1);
float height = pageSize.Height;
float width = pageSize.Width;
We have also created the simple sample which illustrates the same. Kindly check out the sample from the below link: