What is an integer page index?
The integer page index is a 0-based index used to identify pages in a PDF file. The first page has an integer page index of zero.
This indexing system is mainly used in internal PDF data structures and is different from Fragment Identifiers , which use a 1-based counting system.
Why two numbering systems exist side by side
The integer page index is an implementation detail: it is how the page tree inside the file addresses its own pages, and it starts at zero because it is an offset into an array rather than a human-facing number. What a reader sees on screen is something else entirely. That comes from the document's page labels, which can use Roman numerals, letters or chapter-and-page combinations, and which do not have to start at one or run in order at all.
Where the difference matters
The two systems differ by one, and this difference is a common source of off-by-one errors when automating PDF work. A fragment identifier such as the page parameter in a URL counts from one, matching what a person would say, while the internal index counts from zero.
When a tool asks which page to split, rotate or delete, it is almost always asking in human terms. When a library reports a page number, check which of the two it means before you rely on it.