Set download filename to pre last path segment

.htaccess

<IfModule mod_headers.c>
	<IfModule mod_rewrite.c>
		RewriteCond %{REQUEST_URI} ([^/]+)/datasheet.pdf$
		RewriteRule ^.*$ - [E=DATASHEETNAME:%1]
		RequestHeader set datasheetname "%{DATASHEETNAME}e" env=DATASHEETNAME
		Header set Content-Disposition "attachment; filename=%{HTTP_DATASHEETNAME}e.pdf" env=HTTP_DATASHEETNAME
	</IfModule>
</IfModule>

With this .htaccess rule, if the path to your pdf is "http://domain.tld/path/to/my/superduper/datasheet.pdf", the download will get the filename "superduper.pdf".

I work with Apache 2.4 and PHP 7.0 as fcgid. If the file is a static file on the disc (no php involved) or the header is set in the vhost configuration, then the workaround with the RequestHeader is not needed. The pdf is generated dynamically by TYPO3, but the "Header set" didn't work as soon as PHP was called. So it has nothing to do with TYPO3.