Since the game is now several years old, finding the official "free" download can be tricky as online storefronts like Steam have largely moved on to newer eFootball titles.
If you already have PES 2016 in your library, the UEFA EURO 2016 content should appear as an automatic update or a free download in your DLC section. pes-uefa-euro-2016-france-pc-game-free-download
Konami also released a standalone physical version for those who didn't own the base game, which included the full PES 2016 experience plus the Euro 2016 content. Since the game is now several years old,