Having Trouble with Old Sheet Music

It's a long story that begins with a promise I made.

I am currently interning at a historical site that deals in the history of the United States President Theodore Roosevelt. While searching the collection, I noticed that they had old sheet music which I was able to find a PDF of and convert into both TIFF, and PNG along with the original PDF. I have tried a countless different ways of turning the original into a more manageable format for use in other applications. Problem is, each way I do it hasn't ended up working, I've used countless OMR software and each have either spat the product back out at me, shaking their heads no, or have been hidden behind a paywall that stops me from saving the final product upon making it.

If anyone has any suggestions or tips to make this easier, or perhaps could do the transfer for me (since I have little knowledge about sheet music myself. I'm a musician but I am not as well versed in reading sheet music as I used to be) that would be immensely appreciated. I also would rather not spend money on this, since the museum is a non-profit, and as a college student I am not making much money either, so I haven't got $300 to put down on an OMR software that I will use once.

Thank you for your time.

~Connor