The best document format depends on what you want the recipient to do. PDF, Word files, images and scanned documents may look similar on screen, but they are designed for different workflows.
PDF is useful when you want a document to retain a consistent appearance across devices. It is commonly used for forms, reports, invoices, manuals and documents prepared for printing or final distribution.
A Word-processing format is usually better when several people need to revise paragraphs, track changes or continue writing. Converting a complex PDF back into an editable format may not reproduce the original layout perfectly.
JPG and PNG files are appropriate for photographs, screenshots and graphics. They are less convenient for long multi-page documents and generally do not preserve searchable document text in the same way as a text-based PDF.
A scan may look like a normal PDF while each page is actually an image. Search, copy-and-paste and accessibility features may be limited until optical character recognition is applied.
If the recipient only needs to read or print a finished document, PDF is often a sensible choice. If they need to edit it, use the original editable format whenever possible. If the purpose is purely visual, an image may be simpler.