On vacation with no laptop, can’t verify what i’ll be saying.
two suggestions.
for mendeley, see GitHub - tmalsburg/helm-bibtex: Search and manage bibliographies in Emacs.
you might need this: (setq bibtex-completion-pdf-field “File”)
or you might need to make this variable nil to use a standard path resolution logic. experiment with the variable. the documentation suggests that paths created by mendeley would work.
if this does not work, there is an extensive discussion and a solution here