The Church started using the PDF format back in 2001, so you can get PDFs of the Ensigns back to January 2001.
From the
Ensign page, click "Other Issues" and choose a month towards the top, or a different year down below. After choosing a specific Ensign, click the circle with a down arrow in the title bar at the top of the list of articles.
Alternatively, you can go to a PDF directly by following this pattern:
https://media.ldscdn.org/pdf/magazines/ ... gn-eng.pdf
Change where it says january-2001 and 2001-01 to get the Ensign you want.
The New Era, Friend, and English Liahona magazines are also available in PDF format back to January 2001.
Church magazines from before January 2001 are not available in PDF format, only text. They would need to be manually scanned (and there may be copyright reasons for not distributing the older magazines, too – in case there are images, etc. that aren't fully owned by the Church and the license is only for print).