NOTE: This extension is for 'Opencart 2'. The extension for opencart 3 is available here:
https://www.opencart.com/index.php?route=marketplace/extension/info&extension_id=36204

The "read more" extension hides long text descriptions and shows a 'Read more' button above them. When the customer wants to read all of the text they have to click on the 'Read more' button to expand the text. It allows to save a space on a page. It does not impact on SEO because a whole text is pre-loaded on a page.

It should be compatible with many custom themes. I can install it to your store for free.

Demo server
=======================
http://demo2302.ka-station.com/admin/
admin login: demo
admin password: demo

Demo product: http://demo2302.ka-station.com/index.php?route=product/product&product_id=42