File not suitable for content extraction: File contents are too complex for content extraction

Dear Community,

I am going crazy. I am trying to extract a few PDF files and I get the error: "File not suitable for content extraction: File contents are too complex for content extraction" for some files. Unfortunately, this error doesn't help me at all. The files are not large (9 MB), there are only 4 pages and they look exactly the same as all the others I am working on. unfortunately I am not allowed to share the PDF files. Could someone please give me a hint as to WHAT is too complex?

Best regards
Tommy

my code:

try:

    #Initial setup, create credentials instance.
    credentials = Credentials.service_principal_credentials_builder().with_client_id('XXX').with_client_secret('XXX').build()

    #Create an ExecutionContext using credentials and create a new operation instance.
    execution_context = ExecutionContext.create(credentials)
    extract_pdf_operation = ExtractPDFOperation.create_new()

    #Set operation input from a source file.
    source = FileRef.create_from_local_file("XXXX6bfcd6f.pdf")
    extract_pdf_operation.set_input(source)

    #Build ExtractPDF options and set them into the operation
    extract_pdf_options: ExtractPDFOptions = ExtractPDFOptions.builder() \
        .with_element_to_extract(ExtractElementType.TEXT) \
        .with_include_styling_info(True) \
        .build()
    extract_pdf_operation.set_options(extract_pdf_options)

    #Execute the operation.
    result: FileRef = extract_pdf_operation.execute(execution_context)

    #Save the result to the specified location.
    result.save_as("ExtractTextInfoWithStylingInfoFromPDF.zip")
except (ServiceApiException, ServiceUsageException, SdkException):
    logging.exception("Exception encountered while executing operation")