First off yes RUU zip files are encrypted so you can't open them like normal zip files. Not sure about the OTA for E8 as I don't have one yet. OTAs used to be unencrypted but there was talk HTC may start encrypting them also.
To flash an RUU zip with it on the SD card you probably need similar pre-requisites to the HTC One M7 Dual SIM I am familiar with such as: Locked bootloader, zip on external SD FAT32 formatted, not sure if you need to be S-Off if you are flashing the same region/CID version (i.e. x.xx.1402.x for China Unicom).
You probably can flash the RUU using fastboot: fastboot flash zip name_of_zip.zip
again must be same region RUU unless you are S-Off
I must start a new thread and try and collect the different region variations etc and any RUUs and see if it is possible to swap regions like you can on the HTC One M7 - obviously only between RUUs for the same hardware MID. I understand there are single and dual sim E8s in China (M8Sw and M8St respectively??) and the dual sim is also available in Taiwan and Russia. Hopefully the Russian ROM is x.xx.401.x and is a Europe wide ROM. If the MID is the same on all the handsets in different regions swapping ROMs should be possible.