As stated in https://bugs.freedesktop.org/show_bug.cgi?id=64813:

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The Tagged-PDF part of the PDF specification defines rules for documents
to describe their own structure and associated attributes. Having support
for interpreting the structure of documents allows (among others):

- Improve accessibility of documents, e.g. screen readers can use the
  structure to better decide how to handle different kind of elements,
  and to know the logical reading order.

- Reflowing of document content, which is important for better output
  on devices with small screens.

- Better support for exporting to other formats.
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As soon as this support is in Poppler, we need to add corresponding support in Evince.