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