Give a try to pandoc, a text converter.
Efficiency is related to many points (server, memory, size of original file...), so it’s difficult to answer how many ram it will use in your case.
Because it's not possible to answer a simple link, even if by nature, the answer is a link to a software here, I copy-paste the homepage:
If you ...
For the ones that is hosted on your own server and you embed the flipbook within your site, you can use dFlip PDF FlipBook jQuery Plugin.
It is flash free. Costs only $29 for regular license with 6 months customer support. You can create 3D or 2D flipbooks easily with this plugin. Additionally it has awesome page rolling effect.
The company I work for has this (paid) SDK.
See this online demo
There are several on-line choices. The one I use for most of my PDF-related needs has a bunch of conversion/joining functions, each with its own URL. Of those, the one relevant to your question is PDF to DOCX. Depending on your standards/expectations and the complexity of your document, this does a pretty good job.
As I mentioned, there are other choices, ...
Screenplain (the command line version) will convert Fountain files to PDF, Final Draft and HTML.
You need Python and Pip installed ... then just install screenplain and screenplain[pdf].
sudo pip install screenplain
sudo pip install 'screenplain[PDF]'
To use it, just type:
screenplain sample.fountain sample.pdf (or .fdx or .html).
If you ...