converting MCD to PDF or reading MCD files

When you install Prime in the free trial mode you have access to the full features for 30 days before the version falls back to the crippled so called Express version. During this eval time you also can convert legacy Mathcad files to Prime format as Prime can't open legacy files directly.
You have to explicitly check the box for installing the converter when you install Prime.

If your trial period is over you may post the worksheet(s) here and ask if somebody will be kind enough to convert it to Prime format - specify the version of Prime you are using, especially if its not the latest. Don't expect the converted file to be fully working but at least you can see the contents.

There used to be a Mathcad-Viewer many years ago which may still be available somewhere on the net but I don't know if it could read v2001i files - guess no.
It was around Mathcad 7 or 8 which was before 2000 and 2001(i) (which basically were versions 9 and 10). The viewer was called Mathcad Explorer and Vladimir posted some links 10+ years ago: View Mathcad Sheets Without a License - PTC Community

You may also give the file viewer a try which @LucMeekes  kindly made available here. But Luc writes that its a viewer for Mathcad 13..15 files, so it possibly can't handle v2001i files:  Solved: Re: Can't see your sheet, but... - PTC Community

Look like Mathcad Explorer is too old for your 2001i files and Luc's Viewer is too new 😉

The free MathcadBrowser was supplied by Mathsoft only for Mathcad versions 5 and 6. Then they supplied MathcadExplorer for versions 7 and 8, and then it stopped. These were viewers that allowed you to make changes and recalculate an existing sheet or create a whole new sheet, but you could not save the sheet.

My viewsheet application will (try to) show the contents of Mathcad .XMCD files, provided the app that created them is set to save calculation results in the file.

If you had a license 20+ years ago, you had a perpetual license. For version 2001i that (most probably) means it is protected by Cdilla. If you (still) have the installation source (CD-ROM), the next thing you need is a PC running Windows XP, Windows 98, or 95. Then you can install it, license it using the CD (You have to have a CD-ROM player in that PC; to verify your ownership, the software will check the CD for certain sectors), and then run it.

Failing all of the above, you can find someone kind enough to save your files:

- As .PDF files from within Mathcad 11/12/13. This will give the best rendition of the originals, especially if your files make use of the symbolics. (the symbolic engine was changed after Mathcad 13)

- As .PDF files from within Mathcad 14/15..

- As (Mathcad 14/15) .XMCD files (from within Mathcad 14/15) so my viewsheet app can be used to view the contents.

- As Prime files, so you can use Prime Express to look at, and work with, the converted contents, as far as Prime Express will allow.

- As .PDF files from the Prime converter.

Success!

Luc