i ve changed margins (making them smaller) of this one pdf document i opened in Draw but its contents do not snap to the left (new margin). so even when width of the document is the same as its contents the contents are out of the margins. ive tried using “fit object to paper” in page properties but it changes width of the empty areas as well so the width of the contents ends up changing, but height not. the only thing that seems to work is selecting each object on page and setting position X to 0; i can do this with Ctrl+A for all objects on one page, but idk how to select all objects on all pages simultaneously to apply this and since the document is 400+ pages long, going by one page at a time is not doable.
LibreOffice is not a PDF editor.
But specifically in your case, the workaround could be importing PDF into Writer (i.e., open the PDF using File->Open, in the file open dialog select “PDF - Portable Document Format (Writer)” file type, and then select the PDF). Then you have a chance to select everything on all pages, and align them at once.