How to assign the user command in VI(VIM)?

I'm trying to assign the user command in VI(VIM) in Linux(CentOS).

Especially, if there is one script(just text) file which is involved the pdf list file as the below, I want to view the each pdf file by using 'evince' command like as 'gf' command in linux. for example, If I put the cursor on the one of the pdf list, then If I push the function key, then the file which is listed in cursor is executed by using 'evince'.

If I want to see the 3.pdf, then I put the cursor then push the some function key then that pdf file is opened with evince. What am I supposed to do ?

How to assign the user command in VI(VIM)?

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