in the kable package i used a line of code
column_spec(8,width="25em")
This adjustment works relative to the data in the table not relative to the pdf page size. Depending what data is in the table, the table end in the pdf could end in different points throughout.
A few workarounds I can think of is set a width max margin, but don't know a function for that. Rather than something like "25em" set a value that marks the 9 inch mark of where the table should end. Is there something like that available?