Term for the depressions left by a pen tip on an underlying paper sheet

Solution 1:

Impression

• "a mark, indentation, figure, etc., produced by pressure.
• "the act of impressing; state of being impressed"

Solution 2:

Debossing which wikipedia describes as a debossed pattern is sunken into the surface of the material.

Or, indented writing which the Norwich Document Library describes as indented writing, or second page writing, is the impression from the writing instrument captured on sheets of paper below the one that contains the original writing.

Solution 3:

If it's only a faint copy which has to be carefully studied to be read, you can call it a "second page writing". Unlike carbonless copy paper which uses micro-encapsulated dye, a second-page writing relies only on pressure applied on the front sheet. There are a few methods that make the writings clear, and reading a second-page writing, or "indented writing" as mentioned in another answer by E. Frisch, is often helpful to police investigators when such pages are found at a crime scene.

At home, we can try to recover second-page writings (or indented writing)

• applying oblique light to the document
• rubbing a soft lead pencil over its surface

Modern, well-equipped forensic laboratories, however, employ electrostatic detection to recover what was written on the front page.