There are a lot of different factors that can cause this to happen. I don't suppose you are able to post a link to the website that you are having issues with so that we can take a look for ourselves?

Does this only happen in Firefox or does it happen in other browsers as well, such as Internet Explorer or Google Chrome?

Appears in page in IE, not in acrobat reader Cannot provide link, secure internal website

Ok, so the issue is that you want to view the PDF using the Firefox PDF viewer instead of having Firefox open the file using Adobe Reader?

This could be an issue with the default application setting in Firefox. This dictates what programs open each file type. To change the default reader for viewing PDF files in Firefox, follow these steps:

1. Click the menu button and click Options (Preferences for Linux or Mac).
2. In the Options (or Preferences) tab, select the Applications panel.
3. In the Search field, type PDF to find the Portable Document Format (PDF) entry.
4. Use the drop-down menu for that entry under the Action column, to select your favorite PDF reader. To view PDF files in Firefox, choose Preview in Firefox.

For more information, see View PDF files in Firefox or choose another viewer.

No, I want to block this one pdf from automatically opening, others I need, this one is useless

Given that you unable to provide the website or code (which is understandable), this will be a very difficult for us to help solve on this forum.

No worries, I can live with it