PipeDream memory management is automatic. In general, you don't need to worry about memory. When PipeDream needs more memory, it claims it from the system. When PipeDream can release some memory to the system, it does so. In low memory conditions, however, some hints and tips may help:
You may find that you can fit in more data, either in PipeDream or in other applications which are running, if you save all your files, quit PipeDream, then start PipeDream again and load your files again.
The spelling checker makes memory available to PipeDream when it is needed but not to other applications. If you give the Flush Dictionaries command (Ctrl-SF), PipeDream frees all the memory used to store the dictionaries, which may then become available to other applications.
Operating in low memory conditions can be frustrating and cause some applications to crash unexpectedly. It is a good idea to avoid low memory if you can: load fewer documents, quit some applications or, if you can, buy some more memory, or even better, buy a new 32-bit RISC OS system with oodles of cheap memory!