I'm posting this here because I've had this happen to me multiple times and no Google search ever brings up the solution. I'm running Windows 10 with 64-bit MikTex installed and I use TeXnicCenter as my editor/compiler IDE.
I have installed Sumatra and set up LaTex --> PDF workflow with reverse-search. However, instead of opening the source LaTex file in TeXnicCenter, it will open in Notepad++ instead. This is infuriating.
The default command line settings for TeXnicCenter's reverse-search are not escaped properly and therefore will fail to open the instance of TeXnicCenter that launched Sumatra. I suspect it will open your default text editor on other people's machines.
In TeXnicCenter's menu, go to: Build --> Define Output Profiles...
Select your target build profile. In my setup, it is LaTeX => PDF.
Select the Viewer tab.
In the "Executable path" field, my incorrect configuration (set by default installation) was:
C:\Program Files\SumatraPDF\SumatraPDF.exe -inverse-search "\"C:\Program Files\TeXnicCenter\TeXnicCenter.exe\" /ddecmd \"[goto('%f','%l')]\""
Add double-quotes around the first argument so that it looks like the following:
"C:\Program Files\SumatraPDF\SumatraPDF.exe" -inverse-search "\"C:\Program Files\TeXnicCenter\TeXnicCenter.exe\" /ddecmd \"[goto('%f','%l')]\""
Your reverse search should now work properly.