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