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