In the introduction he wrote, “A single open file is a drawish thing...” That's because if there's just one open file the heavy pieces tend to get exchanged and any attempt to win the ensuing balanced minor piece ending is likely to be risky. Any small advantage one might have is not likely to be sufficient to overcome the inherent drawishness. You can see just how drawish a single open file can be in a King's Indian in the embedded game where even the mighty Samuel Reshevsky wasn't able to avoid the draw against a patzer named Tartajubow!!
Purdy's advice was that player trying to win a game with one open file should try to open a second file, but he added the warning...bringing about a second Pawn exchange to open another file without damaging your position can be dangerous. Chess,com has a brief article by Natalia Pogonina HERE.
Purdy's notes to his game against Michael Woodhams in the 1967 Australian Championship are, as are all of Purdy's annotations, quite instructive. Of course, engines punched a few holes in his analysis, but it is the ideas he discussed that are important.
The 1967 Australian Championship was held in Brisbane and was won by Douglas G. Hamilton. Hamilton (born August 15, 1941) is an FM and Correspondence IM. He was also Australian Champion in1964-65 after winning a playoff and again in 1981-82. The 1967 Championship was a 34 player Swiss. Top finishers were:
1) Douglas Hamilton 11.0-4.0
2-3) Max Fuller and Michael Woodhams 10.0-5.0
4-7) Gregory Koshnitsky, C.J.S. Purdy, William Geus and John Purdy 9.5-5.5
8) Stanley Fell 9.0-6.0
Games for Australian tournaments dating back to 1845 are archived at Ozbase.