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!
Father Christmas Puzzle Complete!
Cake Puzzle Complete!
Nativity Puzzle Complete!
Complete Santa Hat
Cat Puzzle Complete!
Popcorn Puzzle Complete!
Cookies Puzzle Complete!
Apples Puzzle Complete!
Gift Puzzle Complete!
Friends Puzzle Complete!
Snowman Puzzle Complete!
Santa Puzzle Complete!
Nut Cracker Puzzle Complete!
Taken Puzzle Complete!
Mince Pies Puzzle Complete!
Dawn Puzzle Complete!
Toy Santa Puzzle Complete!
Bells Puzzle Complete!
Presents Puzzle Complete!
Bear Puzzle Complete!
Socks Puzzle Complete!
Teddy Puzzle Complete!
Gnome Puzzle Complete!
Baubles Puzzle Complete!
Bridge Puzzle Complete!