Trying to open a secured PDF without the password

You can open a secured PDF without the password with Adobe and many other programs. The only way you can open those files is to unlock the PDFs, meaning to crack the passwords you set years ago with some tools. If you google it, you may find some tools that can help you with that.

No Adobe application will allow you to open the file, and the password can't be recovered, either.

What kind of security? An open password?

If you don't have the password to unlock that document is not possible to modify anything that the set restrictions won't allow.

When this happens, your're better off flattening the file (if such restrictions allow you to save as a postscript file , of course) and recreate the PDF from scratch. The Acrobat Distiller won't work  if these restrictions doesn't allow to save as postcript or as image. Printing to PDF may be the only available option which is basically a clean flat PDF layer of the whole PDF with no field objects.

If that doesn't work , then you're probably have to try and print the file as image, then open it in Acrobat to convert to PDF which ultimately brings you back to recreating the form fields and PDF content from scratch as well.