Because a browser on a computer with Adobe Acrobat Reader knows how to open and view a PDF file it will always attempt to display or open the PDF file when the link is clicked.
If you wanted to make it a downloadable file I would suggest you change the extension of the file by compressing the PDF file as a .ZIP file so the user could download and extract the file.
Or a better solution would be to add a comment such as "Right-click the link and choose save as to save the .PDF file." All browsers support the feature to right-click a link and save the link or file to your computer.