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