I have a large document switching several times between portrait and landscape.
I have a horizontal line inside the footer in my portrait format which remains the same in the landscape footer, which is obviously not long enough. I can also not change that manually

Which kind of line have you inserted?

In this case, the easiest solution will be to make use of a top or bottom border to the paragraph in the header. The length of the border will adjust to the width of the paragraph itself.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

That line is likely a border in the paragraph formatting of the footer.

1. Unlink the footer(s) in your landscape version from previous.
2. Then change the right indent for your footer paragraph(s) to match your right margin.
3. That should lengthen the border.

Header/Footer Settings Recap

Assuming that the border is applied to the paragraph (as paragraph formatting), it should adjust.


Unlinking footers should only be necessary if the line was created in a different way, for example if it is a graphic object.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

You are correct. I was thinking that because you are changing paragraph formatting in the footer, if you do not unlink, the new formatting will apply to footers in the previous section(s). While that is true, the left and right indents are relative to the page margins which are adjusted to fit the new layout.