TIP for Firefox users:
To open a pdf file in your browser instead of automatically downloading it, please try the following:
- Open the Firefox Options window.
- Select the Applications tab.
- Locate Portable Document Format (PDF) on the list of file types.
- Change the Action to Preview In Firefox.
- Click Ok.
This should allow you to preview most files in Firefox.
This will not work in all cases because some website contain codes to force the browser to download instead of preview.
Source: https://support.mozilla.org/en-US/questions/985483
For Chrome browsers, this may help.