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