Here is the full list of all 27 Pixel Puzzles 2: Christmas achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Complete All Puzzles
Merry Christmas Puzzle Complete!
Nativity Puzzle Complete!
Father Christmas Puzzle Complete!
Cake Puzzle Complete!
Popcorn Puzzle Complete!
Gift Puzzle Complete!
Apples Puzzle Complete!
Cookies Puzzle Complete!
Cat Puzzle Complete!
Friends Puzzle Complete!
Nut Cracker Puzzle Complete!
Santa Puzzle Complete!
Taken Puzzle Complete!
Snowman Puzzle Complete!
Presents Puzzle Complete!
Toy Santa Puzzle Complete!
Mince Pies Puzzle Complete!
Dawn Puzzle Complete!
Bells Puzzle Complete!
Baubles Puzzle Complete!
Bear Puzzle Complete!
Socks Puzzle Complete!
Teddy Puzzle Complete!
Bridge Puzzle Complete!
Gnome Puzzle Complete!
Complete Santa Hat