the short answer is "it depends" -when you're doing a long-running task such as generating a pdf, the most common reason for a thread abort is that the page times out - the default value depends on the version of .net but is usually 110 seconds, so increasing executionTimeout will often help (shutDownTimeout wont help as it applies to idle applications, not ones running a request).

However there can be other reasons - for instance if the PDF file is very large then it may exceed the maximum size (depending on how it is being generated),  so increasing the maxRequestLength on the HttpRuntime attribute can help.

Another factor can be something external to asp.net -the most common of these is the database e.g if the pdf is being generated based on a database query and it times out (it has a maximum of 90 seconds AFAIR) then the database throws an exception and this bubbles up as a thread abort exception. There's not much that can be done here with the default connectionString as it uses Microsoft's database access library which has a fixed timeout - but you could use your own library (or even just simple ado.net) to set a different timeout and use that (though really the best way is to reduce the load or increase the SQL server ram/cpu)