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