I found this excellent tool: PdfCropMargins, a very light app (both for Linux and Windows), which can crop automatically pdf white margins (top,bottom,left,right) even if they are very irregular (different from page to page). With a great accuracy. And without growing the pdf size.
You can use the gui, starting from a command line, such as pdf-crop-margins -gui “original-pdf.pdf” -o “target-pdf.pdf”.