You don't need to use basic authentication to access that URL and instead should use the DefaultCredential of the SSRS WebService
var theURL = "http://ReportServer/ReportServer_MYSERVER/Pages/ReportViewer.aspx?%2fPurchaseOrder&rs:Command=Render&OrderID=100&rs:ClearSession=true&rs:Format=PDF";
WebClient Client = new WebClient();
Client.UseDefaultCredentials = true;
byte[] myDataBuffer = Client.DownloadData(theURL);
You can also download the PDF using the ReportExecution2005 Web Service. Here is an example:
https://learn.microsoft.com/en-us/dotnet/api/reportexecution2005.reportexecutionservice.render?redirectedfrom=MSDN&view=sqlserver-2016#ReportExecution2005_ReportExecutionService_Render_System_String_System_String_System_String__System_String__System_String__ReportExecution2005_Warning____System_String____
- Under Project --> Add Service Reference -> Advanced -> Add Web Reference -> URL: http:////reportexecution2005.asmx and named it ReportExecution2005.
- Create a method with the following code
ReportExecutionService rsExec = new ReportExecutionService();
rsExec.Credentials = System.Net.CredentialCache.DefaultCredentials;
rsExec.Url = "http://<My Server Name>/<My Report Server Name>/reportexecution2005.asmx";
string historyID = null;
string reportPath = "/<My Folder Name>/<My SSRS Report Name>";
rsExec.LoadReport(reportPath, historyID);
GetProperteriesSample.ReportExecution2005.ParameterValue[] executionParams;
executionParams = new GetProperteriesSample.ReportExecution2005.ParameterValue[1];
executionParams[0] = new GetProperteriesSample.ReportExecution2005.ParameterValue();
executionParams[0].Name = "My Parameter Name";
executionParams[0].Value = "My Parameter Value";
new GetProperteriesSample.ReportExecution2005.ParameterValue();
rsExec.SetExecutionParameters(executionParams, "en-us");
string deviceInfo = null;
string extension;
string encoding;
string mimeType;
GetProperteriesSample.ReportExecution2005.Warning[] warnings = null;
string[] streamIDs = null;
string format = "PDF";
Byte[] results = rsExec.Render(format, deviceInfo, out extension, out mimeType, out encoding, out warnings, out streamIDs);
FileStream stream = File.OpenWrite("c:\\My Destination Folder Name>\\<My PDF Report Name>.pdf");
stream.Write(results, 0, results.Length);
stream.Close();