There are 3 aspects to this:
- the data survives;
- software exists to read the data; and
- hardware exists to run the software.
To ensure the data survives, copy it multiple times, to multiple things (USB drive, DVD, cloud, etc). Run a file comparison program such as diff to check that the copying process worked properly. Media degraded over time, so periodically create new copies.
To ensure software exists, use standard formats such as PDF. Every time you change your computer, make sure you still can open and read the files. If you can’t, talk to whoever produced the old software to get it running on the new computer. Depending on how different the old and new systems are, this could be expensive. However, this also takes care of ensuring that hardware exists to run the software.