This is my understanding:
SDM tomcat is running , otherwise you wouldn't get an HTTP 404 error
pdmweb.exe is a servlet which should be available in tomcat
check if
$NX_ROOT/bopcfg/www/CATALINA_BASE/webapps/CAisd Directory exists.
if not your installation has failed somewhere
if yes provide
$NX_ROOT/bopcfg/www/CATALINA_BASE/webapps/CAisd/WEB-INF/web.xml
Btw. what Version of SDM are you running?
Which OS are you using for SDM Server?
Kind regards
.....Michael