What is actually inside a PDF?
A PDF page looks like one flat thing. It almost never is. What you are looking at is a stack of separate items, drawn in order, each carrying its own instructions.
That is the whole reason two PDF editors can behave completely differently on the same file.
What a page can be made of
- Text, stored as characters with a font, a size and a position — often with the font embedded as a partial copy containing only the characters the page used.
- Images, from a logo to a full-page scan, sometimes shared by several pages at once.
- Links, which are rectangles with a destination rather than coloured words.
- Form fields, separate objects with values, laid over the page.
- Annotations — highlights, notes, stamps, signatures — that live beside the content rather than in it.
- Invisible text, which is how a scan becomes searchable without changing what you see.
- Clipping and shared resources: instructions that show only part of an image, and objects used in more than one place.
None of that is visible as structure. It is visible as a page.
Why editors differ
Because there are two ways to change such a page.
One is to flatten it: read the page, render something that resembles it, and write a new page back. That always “works”, and it quietly replaces things nobody asked to touch — fonts get substituted, an image is recompressed, a form field becomes a picture of a form field.
The other is to find the one object the person meant and change only that. It is harder, it refuses more often, and what comes out is still the document that went in.
The rule we follow
Change what you changed, leave the rest alone.
In practice that means an edit is checked before it is kept: if the rest of the page moved, the edit is not published. When something cannot be done safely, the editor says so instead of producing a page that looks right and is not.
Three shorter pieces cover the practical side of this: editing a PDF without uploading it, changing text that is already in the document, and why a scanned page behaves differently.
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