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