The PDF file that I need to open is a form. The form is only supported by Adobe Acrobat. This implies that I need to open the form with the Adobe Reader.
It is possible to open the Adobe Reader within the Internet Explorer browser if it is configured as a add-on. The other way is to download the file and then open it with the Adobe Reader.
Is it possible for Confluence to open the PDF file in the Adobe Reader rather than the default PDF viewer?