I have just installed Fedora 26.

I have a PDF that contains a ¶ visible paragraph mark but when I copy it into a Libre Office Writer document an 8 appears. So I have to replace the incorrect 8 with a ¶ visible paragraph character

To do this in LibreOffice you click Insert ==> Special Character...

Once you find the character that you want to insert you can:

• Select the character in the Special Characters dialog and click the insert button to put it into your document where the cursor is. Once you have inserted it once you will then be able to copy and paste the special character
• you can also use a keyboard short cut to insert the symbol anywhere within Gnome.

As you can see in the above image what you need is the U+  value which in this case is B6

So once you know that it's simply a matter of pressing CONTROL + SHIFT + u along with the hexadecimal value of the character which is b6

You can also find a map of special characters in Gnome Shell using the "Characters" application

When you click on the Special Character in the "Characters" application a pop-up will allow you to copy the character or give you the Unicode character to use. You can enter CTRL + SHIFT + u b6 or 00b6 and it will work fine.