View pdf file in terminal

You can use sed with binary files (at least GNU sed; some implementations may have trouble with files containing null characters or not ending with a newline character). But the command you used only replaces the first occurrence of /Fit on each line, and lines are pretty much meaningless in a PDF file. You need to replace all occurrences:

 sed s/\/Fit/\/XYZ/g

It would be more robust only replace /Fit if it's not followed by a word constituent (e.g. not replacing /Fitness; I don't know if your file contains occurrences of /Fit that would cause trouble). Here's one way:

perl -pe 's!/Fit\b!/XYZ!g'

This is not a PDF file. This is a Word document. (Well, it's an “Office Open XML” document, but it's Microsoft's format.) These files are zip files under the hood, but the individual files contained in the zip don't make sense on their own.

The person who sent it probably made a mistake when they tried to save it as a PDF, and just renamed the file instead of converting it to PDF. If it's practical, let them know that they actually sent a Word document and try to get them to give you a PDF instead.

You can open Word documents in LibreOffice. Try renaming the file to .docx and your system will probably do that automatically. Usually you can see the text and some of the formatting, but the compatibility is far from perfect. Some elements may be missing or misplaced.