When i create an article i saw that i'm allowed to upload image files only
Allowed file types: .png .gif .jpg .jpeg
How can i add
I'm using zen theme
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.
There is a module for Drupal 6: File attachment utilities; you can get some idea from its code.
Navigate to Administration » Structure » Content types. Click on Manage fields for the "Article" content type. Note that in Drupal 7, the default "Article" content type already have a field for file attachments (named "Attachment"), so there is no need to create a new field, as suggested in the currently accepted answer.
Press the Edit link on the Attachment field and locate the field Allowed file extensions
The default attachment content type is just
txt. Change this to
txt pdf, and press "Save settings".
To add a PDF attachement, make sure you use the Attachment field (not Image field) for the PDF-attachment when creating new Article content.