Opening a PDF brings up the Print dialog

If it's happening only with some specific files then it's probably because:

You can tie Acrobat JavaScript code to a specific PDF document, a page, field, or button within that document, or a field or button within the PDF file, and even to a user action

You can disable javascript in Adobe Reader:

• Open Edit -> Preferences
• Click on Javascript on the left-hand pane
• Uncheck Enable Acrobat Javascript on the right-hand pane

Open in Adobe Acrobat. Goto tools > Javascript > choose "Document Javascripts". Then a dialog box pops up. There will be this.print() code with a corresponding script name (0 in my case). Just press delete botton in the dialog box and save.