This is fairly easy since prestashop uses smarty.
Let's say you want to limit your description to 200 chars and after that display read more button
You can use count_characters. Here is a example:
{if isset($product) && $product->description }
{* full description *}
<div id="idTab1" style="overflow:hidden;height:250px;font-family:avantgarde-book;font-size:12px;line-height:14px;">{$product->description}</div>
{if $product->description|count_characters:true > 200 }
<input id="button" type="button" style="margin-top:5px;font-size:12px;color:white;font-family:avantgarde-book; width:120px;background:#4e3248;border:none;height:30px;border-radius:5px;" value="Mostrar +" onclick="showMore()">
<input id="button2" type="button" style="margin-top:5px;display:none;font-size:12px;color:white;font-family:avantgarde-book; width:120px;background:#4e3248;border:none;height:30px;border-radius:5px;" value="Mostrar -" onclick="showLess()">
{else}
{* don't display anything *}
{/if}
{/if}
You can read about count_characters and about it's attributes from here
http://www.smarty.net/docsv2/en/language.modifier.count.characters.tpl
P.S right way to display something as a comment in smarty syntax is to display it between these tags {* *}
BR's
(if this helps you dont forget to accept it :) )