yes, Pyrochild has explained it completely. if I start PDN manually (by using the program icon), I can open a file (by either the file -> open dialog box, or from windows explorer), close it, and delete the file and it's folder. however, if I do not have PDN open, and I open a file from windows explorer, PDN opens with the file. after I close the file, I am unable to delete the folder that contained the file, as an example I'll be using C:/test/test.png.
[Windows Explorer] open PDN,
[PDN or W E] open test.png,
[PDN] close test.png
[W E] delete C:/test
results in C:/test/test.png DELETED, C:/test DELETED
***
[W E] open test.png
[PDN] (opens with test.png)
[PDN] close test.png
[W E] delete C:/test
results in C:/test/test.png DELETED, C:/test "cannot remove folder test: It is being used by another person or program."
[PDN] exit program
[W E] delete C:/test
results in C:/test DELETED
{edit: replaced backslashes with forward slashes, although it looks weird, but not as weird in the preview, now,,, and removed previous note}