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