In my line of work there are what I like to call the "monsters of the week". These are the problems that have no clear solution and require me to scavenge the bowels of the Internet for an answer. When I find said answer and its cousins, something else inevitably goes wrong. This week involved Internet Explorer 10 not opening PDFs within the browser on Windows 8.
Now, this was a fresh load of Windows 8 Pro (64-Bit) on a brand new machine. I went through the usual of checking the add-ons, reinstalling Reader after performing a complete uninstall, checking Safe Mode, running a virus scan, performing a SFC, etc.
What ultimately fixed the issue was adding a Registry key. A reg key that doesn't exist on any other Windows 8 machines that don't have the issue. The common answer I found was modifying the TabProcGrowth key within HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main. The default value is 0, but needs to be set to 1.
This key also controls IE's ability to drag the tab and create a new window with it. Sure enough, that symptom was also present on the machine I was working on. Again, this key didn't exist on the problematic machine. Nor did it exist on three other machines running Server 2012, Windows 8 Pro (64-Bit), and Windows RT that could open PDFs within the browser.
What fixed the issue for me was creating a new DWORD value within that string, naming it "TabProcGrowth" and setting the value to 1. Closing and reopening IE confirmed I could open PDFs within the browser and create new windows by dragging the tabs.
Since every other Windows 8 machine I tested doesn't have this key and they function fine, I have no clue why it was necessary to add it to this machine. But for those running into the same issue,and there seems to be a lot of you, this is what worked for me. Good luck!
1. Open RegEdit
2. Browse to HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main
3.A. Change the TabProcGrowth value to 1 instead of 0.
3.B. If TabProcGrowth doesn't exist, create a new DWORD value with that name and set the value to 1.
4. Close all running instances of Internet Explorer and launch again.