What is a file format?

A file format shows how its inside is arranged. A picture, a text file, and a PDF can carry the same words, but a program does not read them the same way. An ending such as .pdf or .jpg is a hint.

Changing the ending by hand does not convert the file. Naming a .jpg as .pdf does not turn the picture into a document. To convert it, open the file in a tool that does that job.