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