You mean the download window? That's a client side thing.
If you want a PDF file to open directly, you need to
1: Ensure the client PC has the Acrobat Reader plugin installed.
2: Ensure that the client has set their browser to always open PDFs.
Both of which are client settings, so if this is for something like a local
intranet then you can probably get them changed, but if it's a regular web site
then it's beyond your control.