Portable Document Format — preserves a document's layout, typography, and graphics identically across platforms.
In practice
A PDF packages a document's text, fonts, images and layout so it looks the same on any computer, phone or printer. Designers use it in two main ways: as a proof sent to clients for review, and as a print-ready file sent to the printer, where embedded fonts, bleed and color profile must be correct. A restaurant menu sent as a PDF with embedded fonts keeps its exact typefaces even if the reader does not own them.
Often confused with Composition File
A PDF is a fixed delivery file meant for viewing or printing; the composition file is the layered working document it was exported from.
Try this
Export a document to PDF, open it on a different device, and check that fonts, images and line breaks all match.