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