Thank you for your interesting question.
As you probably know, generating large reports with huge amounts of data is a complicated task. Telerik Reporting and Microsoft Reporting Services are general purpose reporting engines. This means that they are designed to do everyday job in an easy manner - report designer allows you to create a report layout without writing a line of code (or almost without extra coding), preview the report either in Desktop or Web Applications, and export to various document formats.
Your case however is more complicated. To generate a PDF with 5,000 pages requires a lot of processing power and resources and cannot be a fast operation. Up until now we've done some tests with around 1,500 pages which seems to work in a satisfactory way. To our great regret we still haven't got the chance to work on the performance of the reporting engine so it is possible to experience some troubles with larger memory footprint and slower rendering.
To investigate why you get to only 100 pages, we will need to review your application. This will help us locate the problem and give you a resolution. Please, open a new support ticket and attach your report and part of your DB. Thanks in advance.
the Telerik team