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