The safest route is via fiber link between buildings. There is no conductive path since you are running in PVC. Also, no chance of induction (introducing electrical noise on the line) because fiber is non-conductive. No static. No ground problem.
If you are set on running Cat5 or Cat6, you need to be aware of safety while you're doing this...you need to follow the codes of your local jurisdiction. It's fine to save money by doing it yourself, but have you pulled the necessary code requirements? Permits?
It's not only necessary from a legal standpoint, but from a safety standpoint.
From this thread so far, you are creating a non-current-carrying connection between three separate structures. This wire creates a bridge between the buildings that has several potential dangers from lightning and electrical currents going to ground. You must have the three building's electrical systems bonded (not cheap or practical, if they all have their own mains supplied to separate meters). Or, you should not run conductive paths between the buildings.
This is a common Do-It-Yourself safety issue with wiring.
A lighting pulse will travel over your network wire from building to building, and blow out all the equipment in between. Not just one building, but all that are connected.
Differences between ground potential will cause leakage of current from one building to the next, traveling in the direction of best path to ground. You have no control over this. Do you want stray current running uncontrolled over your network wiring and into your network equipment?
Most jurisdiction in the U.S. require pulling of permits, and sometimes on-site inspections of wiring of this nature. The permits can only be pulled by a licensed contractor (general, electrical, or low-voltage) or by the building owner. If it is pulled by the building owner, the building owner is responsible for all insurance, health benefits, wages, and medical care of the persons working on the job. So, any accident or injury is 100% the responsibility of the owner, not the company doing the work.
If you don't own all three buildings, you might find it hard to convince the owners to pull permits for you. Most landlords will not accept financial responsibility for electrical hack work. They will require licensed contractors with $1MM to $2MM insurance coverage.
You also need to pull permits for the trenching and the PVC conduit. There are code requirements for depth, bedding, separation, connection, etc.
Not trying to rain on your parade, but I feel obligated to point out problems that could lead to injury, property damage, or costly fines or repairs.
Since many of the experts here work in the field (i.e., not just hacks), there is a wealth of practical knowledge from real-world experience.
You can always say, "I live in country XYZ" or "I don't care" or "Does not apply". It doesn't hurt anyone's feelings.
Just looking out for your safety.
