Go to C:\Program Files (x86)\Steam\steamapps\common\Splinter Cell - Double Agent\SCDA-Offline\System (alternatively, you can Right Click "Double Agent" in your Steam Library, click Poperties, Local Files tab, then click Browse Local Files)

Scroll down until you find the SplinterCell4 Configuration Settings file.
Right Click and open with notepad.
From here find these lines (Using Ctrl+F) and change the values described below!

SimpleShader=False
SimpleEffect=False
HidePieces=False
bEnableHDR=True
bTrilinearFiltering=False
bSpecularLighting=True
bVerticalSync=True
TextureDetail=High
UseHWShadowMapping=True
ShadowResolutionDegree=0
ShadowTurnOffDegree=0
bUseSmoothShadow=True
FSAADegree=0
AnisotropyDegree=16

Also, Double Agent's maximum supported resolution is 1280x720.
To change this to whatever resolution you want, find these lines.

FullscreenViewportX=
FullscreenViewportY=

For instance X=1920 and Y=1080

That's it! Now enjoy Double Agent the way it was intended to be seen for PC!