Hello Ryan,
In Pro you can change your color scheme to include transparency:
1) Right click on your scanned image in the Table of Contents and select "Symbology"
2) In the Symbology pane, open the drop-down menu for the color scheme
3) Click "Format color scheme" in the drop-down menu
You'll see a black to white color gradient. This is how colors are assigned to your raster.
This is something very useful that you'll likely be using a lot later on, so it's worth giving a bit of background: raster images consist of a set of numerical values. The bit depth determines what type and set of numbers the raster can contain; the most common are whole numbers from 0-255 (8 bit), whole numbers from -32,768 to 32,767 (16 bit signed), whole numbers from 0 to 65,535 (16 bit unsigned), or some low decimal number to some higher decimal number (32 bit float).
What the color scheme does is treat the left side of this gradient as the low end of whatever range of numbers you have, and the right as the high end, and assigns colors depending on where each value falls in that range. So if you were using an 8 bit image, 0 would be black, 255 would be white, and 127 would be gray. Of course, Pro will change the way this assignment works depending on which values your image contains. If your image is 8 bit but only uses 0s and 1s, it will simply set 0 to black and 1 to white instead of using the entire allowed range of 0 to 255. You can customize this in the raster strech settings back in the primary symbology pane.
In any case, according to your legend and the appearance of your scanned image, it looks like your image contains only 0s or 1s. If so, what you want is simpler to get. The markers on either end of the gradient's underside are color stops, which set the colors that the gradient blends to make the gradient.
4) Click on the white color stop.
5) Change the transparency setting to 100%. This will make the white pixels transparent.
6) Since you have a dark base map, you may also wish to set the left color stop to something brighter. Since this looks like mostly roads, a red or orange might work best.
If you follow this procedure with data of higher bit depth, values in the middle of the gradient will be partially transparent pale versions of whatever the left color is, because they are midway between the left color and the right color. This can look bad if you're trying to make it look like the "paper" is transparent; it'll look like someone tried to cut away all the un-inked paper but didn't cut close enough. In such cases, you can get better results by setting both stops to the same color, and just making sure that the right color stop is set to 100% transparency. Note that if you change the color for a stop, its transparency will reset to 0%, so you'll need to change the right stop back to 100% every time you change the color.
If you want to know what bit depth the raster has, you can right-click the raster in the Table of Contents, select "Source", and expand the "Raster Information" section. There should be two entries there called "Pixel Type" and "Pixel Depth".
The following pages have more info on things you can do with this feature:
Color schemes—ArcGIS Pro | ArcGIS Desktop
Custom color ramps for rasters in ArcGIS pro
Map the stars with size, transparency, and color
Hope this helps,
-Charles
