There seems to be a problem when adding a pdf asset from a certain file size upwards. I can select a file of size 28 KB and when i click commit it works fine. But when i select a file of 875 KB and click the commit button i get the standard "The Page Cannot Be Displayed" error page. I haven't tried any file sizes in between to find the exact file size there starts to be a problem with.
i got round this by using the import_files.php script. I also had problems with this, not related. I have mentioned in another topic.
There is a limit to the size PHP will accept. You need to check your php.ini file and increase the file upload and total memory limit in order to upload larger files via PHP.