Why Solr does not index PDF files that are in the same folder?

I have set up a Solr collection in CF 10 Administrator with following settings:

File Extensions: .*

Directory Path: C:/inetpub/wwwroot/test/

Recursively Index Sub Directories: unchecked

Return URL: http://localhost/test/

Language: Default ( I also tried "English" )

Inside the "C:/inetpub/wwwroot/test/" directory, I have 4 different types of files:.xls, .gif, .cfm, and .pdf. But every time when I try to update the index, none of the PDF fiels are indexed. All the other types of documents are indexed. I have looked at the PDF files' properties, none of them are "locked" or "protected".

I am using the Enterprise trial version with all the updates applied.

Anyone has experienced the same issue before?