If you are wondering why your APEX page is not shown in printer-friendly mode after specifying "Yes" in the relevant parameter of the URL, the reason is simply that "Yes" as reported in the documentation of APEX 24.x is wrong, you must specify "YES" in uppercase.
I believe Oracle should accept a case insensitive value for that parameter and avoid a lot of headaches, I mean, it's just a matter of taking the UPPER value and that's it, end of story.
If, for some reason, you cannot change the URL or you don't want to use the URL to activate printer friendly mode, there is still the option to use a conditional before header process where you set the variable:
APEX_APPLICATION.g_printer_friendly := TRUE;
The condition could be based on a REQUEST value or on some other expression of your choosing.
If, for some reason, you are struggling to understand whether the page is in printer-friendly mode or not, the easy way to know it is by adding temporarily a region with the condition "Current page is in printer-friendly mode" (or the opposite, whichever you prefer).
