It depends on the PDF file.

If the PDF file was created from a music notation program, then the PDF is a vector graphic file.

To convert it back to music notation, try PDFtoMusic Pro.

If the PDF was created from an optical scan, then the PDF is a bitmap graphic file.

To convert it back to music notation you need to use a music OCR program such as e. g. SmartScore.

As of now, SmartScore may not work on Catalina. We are expecting a 64-bit version in the next few weeks.

Although SmartScore X2 Pro is supposed to work over Mojave, best to download the demo and see if it works for you. There are issues.

Since Finale owners get a 50% discount on it,

https://www.musitek.com/FinaleUpgrade.html

I'd hold off on purchase till the next version is released—or get an understanding in advance from Musitek that a purchase now would get you the next version at no extra cost. Ask first, don't assume since it's not mentioned on this page. 

https://www.musitek.com/MacNotice.html

As for me, I've got my upgrade money budgeted. I'm waiting for one more app to go 64bit before I upgrade the OS on my work machine to Catalina. In the mean time, I have OS 10.15.4 on an 8 year old MacBook Pro and it runs pretty well.