While working on typographical details it is sometimes useful to compare two different pdf files. Ideally, one version of a document is transparently overlayed onto another for closer inspection. Likewise, there are situations in which one wants to visually verify that the typesetting of two pdf files looks identically. A possible solution for this tasks is provided by the LaTeX package pdfpagediff [1]. However, it is a bit tedious to use, since it requires setting up and compiling a LaTeX document. I was wondering if a much more convenient solution could be implemented in okular. From a user's perspective the workflow would look like this. 1. A pdf file is opened as usually. 2. There is menu button "Compare to..." which allows to select a second pdf file. 3. The second file is displayed as an overlay onto the first file. A slider allows the user to adjust the opacity of the second file. Panning and zooming works as expected. Additionally, a command-line option allows to open two pdf files in such a manner. -- [1] https://ctan.org/pkg/pdfpagediff
I'd like to suggest a much simpler solution to the same problem: I also frequently have to compare two PDFs for minor changes, and I do this by opening both PDFs in okular, each in one tab, and then I flip between both tabs using Ctrl-[ and Ctrl-]. Even tiny differences between the pages become immediately obvious. Such tab-flipping works already very well for comparing single-page documents. But comparing multi-page documents is still tedious: I have to scroll to the next page in each tab separately, and it is very easy for the page selection to get out of sync across the tabs. So it would be extremely helpful if you could just add a new keyboard shortcut "Scroll Page Down All Tabs" = Ctrl-; That new shortcut would scroll to the next page on all tabs (all open files) simultaneously (like pressing space in each tab), such that it becomes easy to stay on the same page in all open documents, for easy comparison. Ctrl-; would be particularly conveniently located on US/UK keyboards, as it is right next to Ctrl-[ and Ctrl-] (Prev/Next Tab), the keys it would be commonly used with.