How can i add .pdf extention on the default upload files
Go to Structure > Content types, click manage fields for the article content type, then edit on the Image field. There you can edit the allowed file types.

However, note that this is an image field. It does not make sense to upload PDF files to an image type field. Instead, you might want to add a new field with type file and configure that to allow pdf files.

Also note that you can't leave the allowed values empty, that would not allow any file types.