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