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