The accept attrubute requires a valid file specifier. It seems for PDF files you need ".pdf".
Upload parameters are specified via the Url property as in our online demo
<RadzenUpload Url=@($"upload/{customParameter}") />
In the example customParameter is a page property of type int. It is accessed in the controller like this:
public IActionResult Post(IFormFile file, int customParameter)
{
}