Buying the series digitally is the most reliable way to own portable copies. These platforms allow you to download episodes directly to your phone or tablet for offline use:
Most major streaming services allow you to download episodes directly to your mobile device or tablet via their official apps. star trek voyager complete series download portable
If you have a subscription, you can download episodes directly to your phone or tablet for "on-the-go" watching without using data. Buying the series digitally is the most reliable