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