backend.php

<?php

public function download()
  {
      $html = request()->data;

      $pdf = App::make('dompdf.wrapper');
      $pdf->loadHTML('<h1>Test</h1>');
      return $pdf->stream();
  }

Vuejs.js

            downloadPlan() {
                let formData = document.getElementById("planComponent").innerHTML

                axios.post('/api/training-plans/download', {responseType: 'arraybuffer', data: formData})
                    .then(response => {
                        this.downloadFile(response, 'customFilename')
                        this.$toasted.show('Nu har nu downloadet planen', {type: 'success'})
                    }, response => {
                        this.$toasted.show('Planen kunne ikke downloades, prøv igen', {type: 'error'})
                        console.warn('error from download_contract')
                        console.log(response)
                        // Manage errors
                    })
            },

            downloadFile(response, filename) {
                // It is necessary to create a new blob object with mime-type explicitly set
                // otherwise only Chrome works like it should
                var newBlob = new Blob([response.data], {type: 'application/pdf'})

                // IE doesn't allow using a blob object directly as link href
                // instead it is necessary to use msSaveOrOpenBlob
                if (window.navigator && window.navigator.msSaveOrOpenBlob) {
                    window.navigator.msSaveOrOpenBlob(newBlob)
                    return
                }

                // For other browsers:
                // Create a link pointing to the ObjectURL containing the blob.
                const data = window.URL.createObjectURL(newBlob)
                var link = document.createElement('a')
                link.href = data
                link.download = filename + '.pdf'
                link.click()
                setTimeout(function () {
                    // For Firefox it is necessary to delay revoking the ObjectURL
                    window.URL.revokeObjectURL(data)
                }, 100)
            },