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