firstPage is a plugin for Zotero 7. It automatically determines
{{firstPage}} from {{pages}} and {{journalAbbreviation}} from {{publicationTitle}}
when you are renaming files automatically.
You can customize the filename format from the General pane of the Zotero settings.
Personally, I like to name my files like so:
{{ authors max="3" join="" suffix="_" }}
{{ journalAbbreviation replaceFrom="[\. ]" replaceTo="" regexOpts="g" truncate="32" }}
{{ volume }}
{{ year prefix="(" suffix=")" }}
{{ firstPage }} {{ title truncate="100" }}
This is intended to provide filenames like
CrawfordStraley_RevSciInstrum92(2021)124707 Improved prescription for winding and electromagnet.pdf
However, the {{firstPage}} and {{journalAbbreviation}} fields are often not populated,
so the template above does not work right away, unfortunately; the result is:
CrawfordStraley_92(2021) Improved prescription for winding and electromagnet.pdf
This plugin tries to help with that: when these fields are empty,
it tries to derive {{firstPage}} from {{pages}}, by splitting it at the hyphen
(or En dash, or Em dash, as appropriate) and keeping only the first part;
and it tries to derive {{journalAbbreviation}} from {{publicationTitle}},
by calling Zotero.Cite.getAbbreviation().
You can install the plugin by downloading firstpage.xpi (using the 'Download raw file' button); then, in Zotero, select Tools→Plugins to open the Plugins Manager window; and drag-and-drop the .xpi on it.