It would be better to add a second drive in the computer. Then you can move
some of the data from the main drive to the second drive. Then you will have
room to install the program for PDF files. You can install the program
either in the first drive or the second drive.
If the computer supports USB, then another possibility would be to move from
data to the flash drive. There might be data in the main drive that does not
need to be there.
Another possibility would be to get the entire copy of the main drive copied
to a bigger drive, but that must be done by someone that is technically
qualified.
For questions about any of this, find a newsgroup or forum or friends to
help you. Since this newsgroup is a Windows Mail newsgroup, I will likely
not provide more assistance with your question here; sorry.