Hi,

Its because the body tag of the pdf report has a class named container-fluid and it has properties 

.container, .o_container_small, .container-fluid, .container-xxl, .container-xl, .container-lg, .container-md, .container-sm {
    width: 100%;
    padding-right: var(--bs-gutter-x, 15px);
    padding-left: var(--bs-gutter-x, 15px);
    margin-right: auto;
    margin-left: auto;
}

So you need to unset the property of padding (which is not recommended)

You could do this to make your custom pdf to your preference

 .container-fluid {
      padding-right: unset !important;
      padding-left: unset !important;
}

If not changing try around it by changing the report type from pdf to html and then click on print pdf and it will redirect you to an html view and change around there by inspecting the view

Hope it helps