What each file stores
An image file is a single rectangle of pixels. JPG, PNG, and WebP all describe one picture at a fixed width and height. There is no page, no text layer, and no layout beyond what was painted into the pixels when the file was created.
A PDF is a document container. One file can hold many pages, and each page can mix text, vector shapes, and embedded images. Programs that open a PDF agree on page size and order, so a report looks the same on your screen, a printer, and someone else's machine.
How each one scales
Pixels are the limit of an image. Zoom past the native width and the picture gets soft, because no extra detail exists to show. Printing does not fix that either; a printer can only place the pixels it was given at the size it was given them.
PDF pages keep their layout and can carry vector artwork that stays sharp at any size. Text in a PDF can remain selectable and searchable. An image has no text unless OCR adds it later, and OCR is a separate step that many tools, Filekind included, do not perform.
When to use which
Send a photo, screenshot, or chart as an image when the receiver only needs to look at it. Send a contract, invoice, or multi-page report as a PDF when page order, paper size, and formatting have to survive forwarding, printing, and other people's software.
The two formats convert into each other easily. Photos become pages in a PDF, and PDF pages render out as JPG or PNG images when you need a picture instead of a document. Both jobs run locally, so the source file never has to leave your device.