So that others may find this, the error message was: "Error from line 39 of /var/www/html/w/extensions/PDFEmbed/PDFEmbed.hooks.php: Call to undefined method Parser::disableCache()". Path may, of course, be different.
In mediawiki version 1.34.4, the file includes/parser/Parser.php had a suggestion in the comments to the function disableCache() to "use getOutput()->updateCacheExpiry()". The function disableCache itself has the line "$this->mOutput->updateCacheExpiry( 0 ); // new style, for consistency" at the end.
I tried, therefore, changing the line "$parser->disableCache();" to "$parser->getOutput()->updateCacheExpiry(0);" in extensions/PDFEmbed/PDFEmbed.hooks.php. This seems to work, but I can't vouch for it's utility or safety.