If you have jdk 1.8 or above installed go to www.executequery.org and download the latest jar file. Download the JDBC driver from Postgres and set it up. It's open source.
It has an ERD generator but obviously depends on having all your foreign keys declared in order to link tables, etc. After generating the ERD you then have to re-organise it by dragging and dropping so that when you print, the diagram is readable.
> Does anybody know a Software for generating graphical entity relation models from existing postgresql databases? > > Best regards Johannes
I use dbWrench (dbwrench.com). It's not free, but they do have a free trial version so you can see if you like it before you buy it. It's also not expensive compared to many of these sorts of tools. It also runs on all 3 major platforms (it's written in Java) and the developer is responsive if you find a problem.
If money is no object, you can look at Power Designer (by Sybase). I used to use it years ago and liked it even if it was MS-Windows only, but the price has gone up so much only companies can really afford it now, IMO.