Here is your test.pdf (images were OCR-ed using Abby Finereader, but with margins left uncropped on purpose) together with two cropped pdfs, using Briss (test_cropped) and k2pdfopt (test_k2pdfopt).
It takes less than a minute (loading, cropping and saving) for both apps if we are familiar with the procedure.
In k2pdfopt we should use trim button (as you did), but together with the graphical mode (for adjusting the cropping rectangle using a mouse/trackpad) and with 0.2 cm blank margin added in the "additional option" field, as shown on the picture (the first box, in the "Crop areas" section, should be checked and the blue button "Select" to be clicked on, for the cropping window to appear).
In Briss (ver. 0.9) we have to load the pdf (using File menu, clicking on OK button when asked whether to exclude/include some pages) and then just to adjust those two blue rectangles (representing a separated & stacked odd and even pages) as close to the text as desired on all the sides, we can then use a preview capability or immediately use a "crop" command under the "Action" menu.
We can also spare ourself some time and have exactly the same size of rectangles by using copy/paste function e.g. by copying the first rectangle, after we have adjusted the first rectangle as desired.
right-click on the second rectangle (right page), and choose "delete rectangle"
right-click on the first rectangle (left page), and choose "select/deselect rectangle" and then after right-clicking on the selected rectangle choose "copy selected rectangle"
right-click on the right (empty) page, choose "paste rectangles" and then move it (position it) over the text as needed.
As k2pdfopt, Briss can also be used for an easy & quick column cropping in multi-column pdf documents or for splitting double-paged (two-page) pdf scans.
https://www.mobileread.com/forums/sh...d.php?t=315771
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Here are some free pdf cropping tools/editors:
https://pdf.iskysoft.com/edit-pdf/cr...-for-free.html
http://www.pdfscissors.tools
online and offline versions, but without valid certificate
https://www.youtube.com/watch?v=fOzh1ixHLa8
There are also quite a few free online services but I've not tried them out on the bigger pdf files (we can split our pdf files if they accept only 50 or 100 pages per pdf file):
https://deftpdf.com/crop-pdf
https://www.sejda.com/crop-pdf
https://avepdf.com/en/crop-pdf
https://pdfresizer.com/crop