How can I add cross references in an existing PDF file?

I have a PDF document that I am reading on my computer running Ubuntu 20.04. The document has over 200 references but, unfortunately, none of the citations of these references are clickable, e.g:

Lorem epsum Quo veritatis tempora asperiores [35] soluta id. Ea similique culpa iusto deleniti rerum vel et. Atque illum sunt voluptate 

The reference [35] is not clickable. My question: What are some tools (preferably open source) that will allow me to create cross-references so in the example above it will allow me to create a hyperlink from [35] to the corresponding reference item in References list (at the end of the document). Your help is very much appreciated

Manually, you would use a good PDF editor (such as Acrobat). With the Link tool, you can create those cross references.

@MaxWyss: Adobe Acrobat is not available in Linux. I found PDF-Xchange editor to be the best alternative because it runs under Wine. but it's not free.