Is the value of the Region field = South West, North Scotland etc. ?
If the value has spaces, normally you'd wrap them in double quotes, otherwise the tabcmd will get to the space and stop...
I wonder if this would work, I haven't tried
tabcmd get "http://126.96.36.199:1111/views/NewYearD/NewYearDashboard.pdf?Region="SouthWest" " -f "C:\AutoTemp\southwestFny.pdf"
Separately, If your query is to pass multiple parameters in a single tabcmd, then the comments in this thread may help
Thanks. It is not working. i have also tried using unicode character for space  , sometimes it goes past and other times it breaks.
Something else might be happening here. I am writing a PowerShell script that gets tabcmd to create the pdf. I am not sure where this is failing, PowerShell side or Tableau tabcmd.
& in the URL has come up before, frustratingly I can't find the thread...
I use tabcmd, but I'm no expert - to try and get you more specialist help I'll move this thread to Server Admin.
For further reference
Many characters such as \ = ( ) do not need to be escaped when they are used within a "quoted string" typically these are charcters you might find in a filename/path. The percent character is one exception to this rule, even though under NTFS % is a valid filename character.
Replace the space with %%20 i.e. tabcmd get "http://188.8.131.52:1111/views/NewYearD/NewYearDashboard.pdf?Region=South%%20West" -f "C:\AutoTemp\southwestFny.pdf"
For removing filter on Region (Region = (All)):
You can try setting "Region=" i.e. tabcmd get "http://184.108.40.206:1111/views/NewYearD/NewYearDashboard.pdf?Region=" -f "C:\AutoTemp\wholeCompany.pdf"