Some .pdn files are experiencing an error while trying to open them

[Dumbuser]

I'm working on a project, and when I went to open it today, it wouldn't open. 
It's a .pdn file.

Last night I saved it, then saved it as a .jpg, printed the .jpg to test and closed the window.

The Jpeg opens fine, but it's opening as the previous version rather than the most recently saved version.

I have already tried renaming the file which did not resolve the issue. I have also attached the guilty file to this post I am "not permitted to upload this kind of file".

This is the text from the error message: 

 

"System.FormatException: file is not a valid paint.net document

 

   at PaintDotNet.Document.FromStream(Stream stream) in D:\src\pdn\src\Data\Document.cs:line 1328

 

   at PaintDotNet.FileType.Load(Stream input) in D:\src\pdn\src\Data\FileType.cs:line 496

 

   at PaintDotNet.Functional.Func.Eval[T1,TRet](Func`2 f, T1 arg1) in D:\src\pdn\src\Base\Functional\Func.cs:line 158

"

I'd like to know how to fix the error as well as how to prevent the error in the future because it has happened to me twice now in the week I've been using the program.