I am a newbie in terms of WordPress and related plugins. I need to create a search page where the user can type in a serial number. The search engine displays only the corresponding certificate of analysis (pdf file) which can be downloaded by the user, if necessary.

With “standard” search engines, all kinds of files are displayed, including pages, posts, attachments, etc. I would like only specific files to be searched (those that I want to be searched, for instance, those that are placed in a specific file). Also, a result should appear only if the serial number is existing. There should be only one result by search at max (only the searched file), to avoid unwanted downloads.

Also, a free plugin would preferable of course…

I have tried a few plugins so far, but either the search engine did not allow the wished specificity, replaced the site search engine*, or was too expensive…

Some guidance would be highly appreciated.
Best wishes for this end of year 2019.

*I want a plugin that is in addition to the site engine, or one that allows various search options.

Check out Ajax Search Pro, https://www.ajaxsearchpro.com. At $36 I think it is well worth the investment. I use the plugin to search advanced custom fields but it also supports PDF searches.