A PDF can be reflowed without reconstructive processing only if a PDF was generated as a Tagged PDF [1] and if the viewer supports reflowing.
[1]: Essentially a PDF with its own EPUB inside it, but unlike just having an attached EPUB, there is a map between the page layout of the PDF and the tags.
There are implementations of reconstructive reflowing that infer the layout block structure and reading order and can reflow a two column paper into a single column.
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A PDF can be reflowed without reconstructive processing only if a PDF was generated as a Tagged PDF [1] and if the viewer supports reflowing.
[1]: Essentially a PDF with its own EPUB inside it, but unlike just having an attached EPUB, there is a map between the page layout of the PDF and the tags.
There are implementations of reconstructive reflowing that infer the layout block structure and reading order and can reflow a two column paper into a single column.