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