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