PDF-XChange does a lookup in the registry to find the executable named in InstallPath for Adobe Acrobat Reader.
On Windows 7-x64 PDFXEdit looks in HKLM, but not the 32-bit keys for Readers-Installpaths.
PDFXEdit looks in HKLM-64bit keys or in USER-keys, but in the USER-keys often the InstallPath key is missing (maybe Adobe´s fault, but that's the case
After install of Acrobat Reader the following key/value is present:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Adobe\Acrobat Reader\10.0\InstallPath
"@"="C:\Program Files (x86)\Adobe\Reader 10.0\Reader"
If you create this value in HKEY_LOCAL_MACHINE\SOFTWARE\Adobe\Acrobat Reader\10.0\InstallPath it will work.
Of course this must be fixed by Tracker in a future builds of this great product!
