A pde file extension is associated with the Processing programming language. A pde file stores source code. Probably obsolete file format.
PDE file extension - Processing development environment source code
What is pde file? How to open pde files?
File type specification:
The pde file extension is related to the source code written in Processing programming language.
Processing is an open source programming language and environment for people who want to create images, animations, and interactions. Initially developed to serve as a software sketchbook and to teach fundamentals of computer programming within a visual context, Processing also has evolved into a tool for generating finished professional work.
This pde file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.