What does pp mean in a letter signature?

This comes from the Latin 'per procurationem' which means by delegation to, or through the agency of. It is used when signing documents on behalf of someone else.

This is regularly used when a secretary signs a letter on behalf of the person they work for.

For example:

Yours,

pp. (Then signature here)

John Smith

Head of Operations

This would mean someone working for John Smith wrote/finalised the letter and signed it.

Hope this helps