Fot that functionality is use this widget: https://appstore.home.mendix.com/link/app/12/
It can show the pdf inline or in a pop-up.
Keep that file in your mendix project directory and try again.
Dear Anasa,
Browsers can not access local files from disk, for good security reasons. You can only link to web URLS.
If you want users to access PDFs, make sure they are store in a System.FileDocument and use de Download activity of a microflow (https://docs.mendix.com/refguide/download-file) or the download button for the File Manager https://docs.mendix.com/refguide/file-manager
Alternative you can use a widget from the app store to display the PDF in your application
Cheers, Andries