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