Chrome to open an “open file dialog” when downloading file?

To the best of my knowledge and understanding this is due to browser plug-ins. For example, if a browser is capable of showing .pdf, it will just show it in the browser window and will not show Save dialogue box.

Technically, you cannot (and should not) force the user to save the file. The user should be smart enough to know what is happening and what s/he wants.

If you wish to force it, you should .zip the files on server side and send a .zip file and not the .pdf or .xls file.

.zip will definitely show save dialogue box.