If I place a
10" X 6.7" 72 ppi
.jpg into InDesign and scale the image down to about
3" X 2", does the ppi change for that image?
And how can I tell?
You are "stuffing' the same amount of pixels into a smaller width and height. Doing this increases the density of the pixels.
To see the effective ppi after the reduction, simply select the image/object and look at the Info Panel (
Window > Info).
If you're going to reduce low resolution images in Indesign, it may be helpful to remember that all 72ppi images must be reduced to a minimum of 30% their original size to achieve an effective ppi suitable for printing (typically 240-300 dpi [or 1.5x the line screen to be used by the press]).
It is generally best practice to configure raster images to be imported into Indesign as they will be output. You really should set up the raster images so you can import them at 100% and without any rotation, reduction, or enlargement, in Indesign. This ensures better output.