Problem Description
We added Bookmark in Okular, but it cannot be displayed in Foxit Reader 2.4.4 and Evince 3.30.2.
At first we thought that the bookmarks were maintained by the software itself, but then we found that the PDF file was indeed modified, so it can be excluded. Then we looked up the PDF specification and looked up relevant blog posts that explained the problem.
This note will explain the problem and add additional explanations. Note that depending on the context, we may use Bookmark to refer to "bookmark" and Outline to refer to "outline".
problem causes
In the PDF specification, Bookmark and Outline have been merged and collectively referred to as Outline, as described in the PDF specification:
There is no clear specification for Bookmark, so PDF has its own implementation, so the software behaves differently (just like the problem we encountered).
Bookmark and Outline
What is a bookmark (Bookmark)?
It can be likened to bookmarks in the real world, marking them, and then allowing us to quickly find certain pages in the book.
What is Outline?
The outline is similar to an abstract in a book, usually at the beginning of the chapter, so we can quickly understand the content of the book (PDF). This "summary" is probably small text description , it could be a content list (Table of Contents)
Why are the outline and table of contents merged?
First of all, we must clarify two issues:
1) PDF is a document format, without the concept of "table of contents". The table of contents we see is artificially added marks (bookmarks, outlines)
2) The outline is not necessarily a text description, it can Is an overview list of chapter contents (as in the "ToC" block in the upper right corner of this article)
Now back to our question. In the process of reading PDF documents, we will see what we are interested in, and then add Bookmark for detailed reading later. So the question is, should the mark we made appear in Outline? Some people think that this is that Bookmark should not appear in Outline, but some people think that the marked content is an integral part of the content of the document and should appear as an Outline and as a summary.
Different perspectives and lack of specifications lead to different software implementations.
Questions to consider when choosing PDF reading software
If we need to edit PDF documents frequently and review them, then we may need to choose a fixed PDF software (cannot be changed frequently, basically cannot be changed) , otherwise, bookmarks and marks may not be viewed.
If we need to read PDF documents frequently, there are no too many restrictions.
references
书签 与 大纲（Bookmark and Outline）
PDF - Wikipedia
Understanding the Difference between Bookmarks and the Outline in a PDF | Inside PSPDFKit