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