1. It can't be helped, but you might tend to find debris that were invisible before you scanned but sticks out like a sore thumb after adjusting the levels.

You can erase them with the Eraser Tool (Press "E") and click the mouse on the areas you'd like to erase.

 

Troubleshooting.

-"Instead of erasing, I'm filling the areas with black/some colour."

Take a look at the two larger squares somewhere near below in the toolbar above. The eraser fills the erased area according to the lower square, or known as the Background colour. Make sure that the background colour is white. You can interchange the Background colour with the Foreground colour (the higher square) by pressing "X" on the keyboard. This is sometimes done accidently by beginners.

Regardless of what are the current background and foreground colours, you can revert them back to black (foreground) and white (background) by simply pressing "D".

 

-"I'm using a weird eraser that does weird things."

Press the Enter button to open the options window for your current eraser (If it's not there). Make sure it's not "Magic Eraser" or "Background Eraser",etc. If it is, press Shift+"E" to toggle through the available erasers in Photoshop.

Make sure it's just a normal eraser.

 

-"My eraser's too big/ too small."

Adjust your eraser by selecting the available sizes in the Brush Window. If you can't see it, open it by selecting Window, > Show Brushes. You can quickly show or hide it by simply pressing F5 on the keyboard.

 

-"I can't see my eraser, I only see a crosshair."

Make sure the Caps Lock button is not on.

 

2. Alternatively, you can select what you'd like to erase by using a Lasso to create a selection and press the Delete button. You can select it by pressing "L" on the keyboard. Move the mouse around the desired areas and make the ends meet to create a selection. If you have poor control with the mouse, I recommend switching to the Polygonal Lasso by pressing Shift+"L".

The lasso helps so you'll only erase the selected areas and not accidently erase other parts of your drawing. Note that I made multiple selections by holding the Shift button while using the Lasso.

 

3. Now you can save your image (unless you've decided to do it sooner). For your info, my disk space-friendly size is around 1.5 MB.