string strPDF = @"http://Myserver/MyPdf.pdf";
HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(strPDF);
HttpWebResponse myResponce = (HttpWebResponse)myRequest.GetResponse();
PdfDocument inputDocument1;
try
{
     inputDocument1 = PdfSharp.Pdf.IO.PdfReader.Open(myResponce.GetResponseStream(),PdfSharp.Pdf.IO.PdfDocumentOpenMode.Import );
}
catch(Exception ex)
{
     lblMessage.Text = ex.ToString();
     return;
}

string strPDF = "http://MyServer/MyPdf.pdf;

byte[] buffer = new byte[4096];

WebRequest wr = WebRequest.Create(strPDF);
               
WebResponse response = wr.GetResponse();

Stream responseStream = response.GetResponseStream();
MemoryStream memoryStream = new MemoryStream();
                       
int count = 0;
do
{
   count = responseStream.Read(buffer, 0, buffer.Length);
   memoryStream.Write(buffer, 0, count);

} while (count != 0);

PdfDocument inputDocument1;
try
{
   inputDocument1 = PdfSharp.Pdf.IO.PdfReader.Open(memoryStream, PdfSharp.Pdf.IO.PdfDocumentOpenMode.Import);
}
catch(Exception ex)
{
   lblMessage.Text = ex.ToString();
   return;
}