To fix this issue download and install the latest version of RapidAuthor (starting from 6.3).
If installing the newest version of RapidAuthor doesn't solve the issue, then you could increase the size of RAM usage for PDF publishing in the file "C:\Program Files\Common Files\ParallelGraphics\ApacheFOP\fop.bat"
RapidAuthor 6.3 should already include this line at the bottom of the file: "%JAVACMD%" %JAVAOPTS% -Xmx512m %LOGCHOICE% %LOGLEVEL% -cp "%LOCALCLASSPATH%" %FOP_OPTS% org.apache.fop.cli.Main %FOP_CMD_LINE_ARGS%.
If you do not have RapidAuthor 6.3 yet, then you could simply add this line to the fop.bat
If the error still persists, 512 in "-Xmx512m" can be increased to 1024 or more.