need to exclude certain files

Seems like the easiest way to do this would be to add in a second variable that keeps track of if the file that is being requested is a pdf, if I am understanding your requirements correctly. I have not tested this iRule but I believe it should look something like this.

when HTTP_REQUEST { 
    set asm_404_not_found 0 
    if { [HTTP::uri] ends_with ".pdf"} {
        set pdf_requested 0
    }
}
when ASM_REQUEST_BLOCKING 
{ 
    set asm_info [ASM::violation_data]  Any response tweaking should only be done in blocking mode!!! 
    if {[string compare [ASM::status] "blocked"] == 0} { 
        if {[string first {VIOLATION_OBJ_DOESNT_EXIST} [lindex $asm_info 0]] != -1} { 
            set asm_404_not_found 1 
            if {[info exists pdf_requested]} {
                set pdf_requested 1
            } else {
                ASM::disable 
                TCP::close 
            }
        } 
    } 
}
    
when HTTP_RESPONSE_RELEASE { 
    if {$asm_404_not_found == 1} { 
        if{$pdf_requested == 1} {
            HTTP::respond 301 Location "https://www.site.com/otherpage.aspx"
        } else {
            HTTP::respond 301 Location "https://www.site.com/404.aspx"
        }
    } 
}

For future reference, you can put formatted code into devcentral without an issue if you put "~~~" above and below the code you want.

Hope this helps. If you have any more questions, I am sure I can help.