Not all printers can print a paper up to its edges. Most of them leave an unprinted margin.
LibreOffice offers a semi-automatic feature that enables you to print as close to the paper's edge as is possible.
Make sure that your printer has been setup under \<emph\>File - Printer Settings\</emph\>.
Make sure that the Web in the View menu is not selected.
Select the \<emph\>Format - Page\</emph\> command, and go to the \<emph\>Page\</emph\> tab.
Under \<emph\>Margins\</emph\> you can define the maximum or minimum possible value for the page margins (left, right, top, and bottom). Click into the respective spin box, then press the (Page Up) or (Page Down) key. The preview displays a dashed line around the printable range.
Click \<emph\>OK\</emph\> to close the dialog.