Generated PDFs do not have bookmarks in R2 2021 unless there is a TOC section in the report.
By setting the device info setting "ProcessItemActions", the generated PDFs have bookmarks again.
var reportProcessor = new Telerik.Reporting.Processing.ReportProcessor();
var reportSource = new Telerik.Reporting.UriReportSource();
reportSource.Uri = "some Uri";
var deviceInfo = new System.Collections.Hashtable();
deviceInfo["ProcessItemActions"] = true;
reportProcessor.RenderReport("PDF", reportSource, deviceInfo);