p.p.
Per procurationem (used when signing a letter on someone else’s behalf).
[Latin]
Usage
The traditional way to use pp when signing a letter on someone else’s behalf is to place pp before one’s own name rather than before the name of the other person. This is because the original Latin phrase per procurationem means ‘through the agency of’. However, pp is now often taken to mean ‘on behalf of’ and is placed before the name of the person who has not signed, and this has become standard practice in many offices.
http://www.oxforddictionaries.com/definition/english/pp


A common usage of per procurationem in the UK and other Commonwealth countries occurs in business letters, which are often signed on behalf of another person. For example, given a secretary authorized to sign a letter on behalf of the president of a company, the signature takes the form:

p.p. Secretary's Signature
President's Name

or

President's Name
p.p. Secretary's Signature

Commonly in practice, an alternative form is used:

Secretary's Signature
p.p. President's Name

https://en.wikipedia.org/wiki/Procuration

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Note added at 3 hrs (2016-01-30 18:12:07 GMT)
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M. Gačić, Englesko-hrvatski rječnik prava:
per pro (per procurationem) - po ovlaštenju
sign per pro - potpisati po ovlaštenju