Most people don't use their Z88 as their main computer, but rather as a portable complement to their desktop system. So, one of the most important features of the Z88 is its ability to transfer data with those other computers.
If you're handy with a soldering iron, or know someone who is, you can easily make up your own cable to connect to your computer's serial port. Here are ones which are know to work:
Once you have the cable, you'll need some software to perform the file transfers.
Plain text files, including PipeDream and CLI files can be transferred in a variety of ways, without the need for extra software on your Z88 - although you may need a terminal program for your other computer. Follow this link for details.
Binary files (such as BASIC programs) are more problematic. You could either write a program for your computer to support the Import/Export popdown protocol (details on p93 of the user guide), or make use of standard terminal software at each end. Two good Z88 terminal software packages are Z88COM and ZCP - follow these links for the software & details of how to use it.
Alternatively, a freeware PC application (by Anton Bialecki) which links directly to the Import/Export popdown is available, called (originally enough) Z88.COM. Instructions are now available in both French and English.