Machine.php

class Machine extends AbstractEntity
{
  /**
     * pdf
     *
     * @var \TYPO3\CMS\Extbase\Domain\Model\FileReference
     */
    protected $pdf = null;
    
    /**
     * Returns the pdf
     *
     * @return \TYPO3\CMS\Extbase\Domain\Model\FileReference $pdf
     */
    public function getPdf()
    {
        return $this->pdf;
    }

    /**
     * Sets the pdf
     *
     * @param \TYPO3\CMS\Extbase\Domain\Model\FileReference $pdf
     * @return void
     */
    public function setPdf(\TYPO3\CMS\Extbase\Domain\Model\FileReference $pdf)
    {
        $this->pdf = $pdf;
    }
}

RestController.php

 class RestController extends ActionController { 
    public function filterSledPositionsAction() {

        if($this->request->hasArgument('machine')) {
            $machineUid = $this->request->getArgument('machine');
            
            /** @var \Deichbrise\GoeltenbodtProducts\Domain\Model\Machine $machine */
            $machine = $this->machineRepository->findByIdentifier($machineUid);

            var_dump($machine->getPdf()->getOriginalResource()->getOriginalFile()->getPublicUrl());die();

            $this->view->assign('value', $machine->getSledPosition()->toArray());
        } else {
            $this->view->assign('value', null);
        }
    }
 }