text: Title: [BUG] Merging PDF + PNG omits PNG
  Body: ### Description
  
  When attempting to merge a PDF-based document with another document that resulted from a PNG upload the merge process finishes without reporting any obvious error, but the resulting "merged" PDF is missing the page from the PNG-based document. No error is visible from the web frontend.
  
  Working around the issue by downloading and re-uploading the PDF generated for the PNG (and using that for the merge) unfortunately does not work as it is rejected as a duplicate.
  
  ### Steps to reproduce
  
  1. Upload PDF
  2. Upload PNG
  3. Select both documents for merging (generate new metadata, do not delete original files).
  
  ### Webserver logs
  
  ```bash
  [2024-08-27 10:44:43,663] [INFO] [paperless.bulk_edit] Attempting to merge 2 documents into a single document.
  [2024-08-27 10:44:43,820] [ERROR] [paperless.bulk_edit] Error merging document 592, it will not be included in the merge: /usr/src/paperless/media/documents/originals/0000592.png: unable to find trailer dictionary while recovering damaged file
  Traceback (most recent call last):
    File "/usr/src/paperless/src/documents/bulk_edit.py", line 262, in merge
      with pikepdf.open(str(doc.source_path)) as pdf:
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/usr/local/lib/python3.11/site-packages/pikepdf/_methods.py", line 398, in open
      pdf = Pdf._open(
            ^^^^^^^^^^
  pikepdf._core.PdfError: /usr/src/paperless/media/documents/originals/0000592.png: unable to find trailer dictionary while recovering damaged file
  [2024-08-27 10:44:43,822] [INFO] [paperless.bulk_edit] Adding merged document to the task queue.
  [2024-08-27 10:44:43,857] [INFO] [celery.worker.strategy] Task documents.tasks.consume_file[fa0ac878-d0ad-472f-9f8b-b1a6f934d58a] received
  [2024-08-27 10:44:43,905] [INFO] [paperless.tasks] WorkflowTriggerPlugin completed with: 
  [2024-08-27 10:44:43,913] [INFO] [paperless.consumer] Consuming 618_592_merged.pdf
  [2024-08-27 10:44:43,931] [INFO] [paperless.parsing.tesseract] pdftotext exited 0
  [2024-08-27 10:44:45,693] [INFO] [ocrmypdf._pipeline] page is facing ⇧, confidence 5.52 - no change
  [2024-08-27 10:44:55,983] [INFO] [ocrmypdf._pipelines.ocr] Postprocessing...
  [2024-08-27 10:44:56,807] [INFO] [ocrmypdf._pipeline] Image optimization ratio: 1.22 savings: 18.3%
  [2024-08-27 10:44:56,808] [INFO] [ocrmypdf._pipeline] Total file size ratio: 1.24 savings: 19.6%
  [2024-08-27 10:44:56,810] [INFO] [ocrmypdf._pipelines._common] Output file is a PDF/A-2B (as expected)
  [2024-08-27 10:44:58,394] [INFO] [paperless.parsing] convert exited 0
  [2024-08-27 10:45:01,713] [INFO] [paperless.handlers] Assigning correspondent Thilo-Alexander Ginkel to 2024-08-26 618_592_merged
  [2024-08-27 10:45:01,722] [INFO] [paperless.handlers] Assigning document type Abrechnung to 2024-08-26 Thilo-Alexander Ginkel 618_592_merged
  [2024-08-27 10:45:01,732] [INFO] [paperless.handlers] Tagging "2024-08-26 Thilo-Alexander Ginkel 618_592_merged" with "Veranstaltung, Abrechnung"
  [2024-08-27 10:45:01,799] [INFO] [paperless.consumer] Document 2024-08-26 Thilo-Alexander Ginkel 618_592_merged consumption finished
  [2024-08-27 10:45:01,803] [INFO] [paperless.tasks] ConsumeTaskPlugin completed with: Success. New document id 619 created
  ```
  
  
  ### Browser logs
  
  _No response_
  
  ### Paperless-ngx version
  
  2.11.6
  
  ### Host OS
  
  Ubuntu 22.04
  
  ### Installation method
  
  Docker - official image
  
  ### System status
  
  ```json
  {
      "pngx_version": "2.11.6",
      "server_os": "Linux-5.15.0-117-generic-x86_64-with-glibc2.36",
      "install_type": "docker",
      "storage": {
          "total": 473533612032,
          "available": 359678787584
      },
      "database": {
          "type": "postgresql",
          "url": "paperless",
          "status": "OK",
          "error": null,
          "migration_status": {
              "latest_migration": "documents.1052_document_transaction_id",
              "unapplied_migrations": []
          }
      },
      "tasks": {
          "redis_url": "redis://broker:6379",
          "redis_status": "OK",
          "redis_error": null,
          "celery_status": "OK",
          "index_status": "OK",
          "index_last_modified": "2024-08-27T10:47:27.021629+02:00",
          "index_error": null,
          "classifier_status": "OK",
          "classifier_last_trained": "2024-08-27T08:05:00.617726Z",
          "classifier_error": null
      }
  }
  ```
  
  
  ### Browser
  
  _No response_
  
  ### Configuration changes
  
  _No response_
  
  ### Please confirm the following
  
  - [X] I believe this issue is a bug that affects all users of Paperless-ngx, not something specific to my installation.
  - [X] I have already searched for relevant existing issues and discussions before opening this report.
  - [X] I have updated the title field above with a concise description.