Drupal 8.x

Disclaimer: I am a skillful front-end user who can tinker with structure sometimes - but I'm now over my head. 

I've been hired by an association that had someone set up a dev site for them, and disappeared without leaving any documentation or support for the build out.

My current struggle is with resource wrappers. This site has 3 media types defined - document, image, and resource. Neither Document or Resource permit a link to an existing doc - which is really what I want. Their URL will not change, and several hundred docs are already uploaded to their site.

Got a new clean setup install of 8.5.1

When I click on the " Clear all caches " button, I get a blank screen and have to go back.

To clarify the rest of the site works fine and going back works fine.

Just a glitch I would like to resolve.

The Browser Console shows:

Hi guys. Hope you can help me.

I use drupal 8.

I want pdf preview like this http://prntscr.com/j08ljo.

Need uploud pdf file in node and showing just first image in front page.

I check use module pdfpreview https://www.drupal.org/project/pdfpreview

Have with tutorial and dependence. Setup ImageMagick and configured it in image toolkit.
Sent in server files with imagemagick and write path.

apologies for posting a dummy's question, but I am not a developer but small business owner.  I have successfully built drupal 7 sites entirely with the drupal web interface and e.g. filezilla, and am just trying to get to grips with Drupal commerce in Drupal 8. I keep seeing references to composer starting with for example "run composer install" with in the commerceguys kickstart package - but where do I find the place to actually type that? 

My firm has developed a site in Drupal 8 and we are ready to host our advertising. We don't use google ads, we have our own clients with and sell our own ads. Hence we need a platform that will host and manage our own ads. Not many of these modules have been upgraded for drupal 8, though it may be in the process. Anyone know a good plugin to recommend for what I am looking to do? 

Here's a situation that I'm not understanding about how Drupal routes are instantiated. I'm using this general form, in particular the SponsorbuildDeleteForm.php class in my_module/src/Form/SponsorbuildDeleteForm.

https://medium.com/valuebound/how-to-create-custom-form-with-crud-create...