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