Worker for this variable is usually a census definition based of economically active population. More information is available at http://pwt.econ.upenn.edu/Documentation/append61.pdf.
For proper citation, see http://pwt.econ.upenn.edu/php_site/pwt_index.php
Source Indicator: rgdpl2wok
Source: University of Pennsylvania
Release: Penn World Table 7.1
University of Pennsylvania, Purchasing Power Parity Converted GDP Laspeyres per worker for Bulgaria [RGDPLWBGA627NUPN], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/RGDPLWBGA627NUPN/, July 1, 2015.