How can I export the content of pdf-forms into a .csv-file?

Unfortunately the answers so far don't really help to really answer the question.

What you really need to do in order to get a predictable CSV from your PDF form field data, having full control, is the following:

Create a JavaScript which you can start, for example, by using the "Option" area of a "Button". (All JavaScript-related information can be found here: https://opensource.adobe.com/dc-acrobat-sdk-docs/library/jsapiref/JS_API_AcroJS.html .)

Within this JavaScript, you iterate through all form fields using eg:

   for ( i = 0; i < this.numFields; ++i ) {
      theField = this.getField(this.getNthFieldName(i));
      // acquire eg the Field's name and value and write it to a string (9 = TAB; D = Return):

      myCSV = myCSV + theField.name + "\u0009" + theField.value + "\u000D";

      ...

   }

Then, you create a data object and have it exported, having opened a dialog window to let the user choose the CSV's location, eg like this:

   this.createDataObject("output.csv", myCSV);

   this.exportDataObject({ cName:"output.csv", nLaunch:"1"});

I hope you can get the principle and now tailor you CSV to your needs.

To export it as a CSV file you would need a script, but you can export it as a tab-delimited text file.

Follow these instructions: Collect and manage PDF form data, Adobe Acrobat