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