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