Resolution of PDF

PDF files don't have a resolution. They can contain images of various resolutions without issue.

You cannot do anything to change a low rez to a high rez in Acrobat. If you can pull that image out of Acrobat and increase the resolution in PS or Topaz software, then you can put it back. But if you are unfamiliar with the process, there might be a bit of a learning curve. But that's OK, learning new things is always a great thing.

What is the issue. 300dpi is no holy cow. It's anyhow only relevant if you have pixel content.