Remove last vestiges of command line usage of qpdf - change to check_pdf

docs/advanced.rst

@@ -148,7 +148,8 @@ In addition to tesseract, OCRmyPDF uses the following external binaries:
 
 -  ``gs`` (Ghostscript)
 -  ``unpaper``
--  ``qpdf``
+-  ``pngquant``
+-  ``jbig2``
 
 In each case OCRmyPDF will search the ``PATH`` environment variable to
 locate the binaries.

docs/errors.rst

@@ -32,10 +32,10 @@ As the error message suggests, your options are:
 Input file 'filename' is not a valid PDF
 ========================================
 
-OCRmyPDF passes files through qpdf, a program that fixes errors in PDFs,
-before it tries to work on them. In most cases this happens because the
-PDF is corrupt and truncated (incomplete file copying) and not much can
-be done.
+OCRmyPDF checks files with pikepdf, a library that in turn uses libqpdf to fixes
+errors in PDFs, before it tries to work on them. In most cases this happens
+because the PDF is corrupt and truncated (incomplete file copying) and not much
+can be done.
 
 You can try rewriting the file with Ghostscript:
 

docs/release_notes.rst

@@ -30,6 +30,7 @@ v10.0.0 (not yet released)
    ``ocrmypdf.helpers.Resolution`` class.
 -  A deprecated parameter in ``ocrmypdf.exec.ghostscript.generate_pdfa`` was
    removed.
+-  The deprecated module ``ocrmypdf.exec.qpdf`` was removed.
 -  The ``ocrmypdf.hocrtransform`` module has been updated to follow PEP8 naming
    conventions.
 

docs/security.rst

@@ -68,7 +68,7 @@ license, OCRmyPDF's GPL license, and any other licenses.
 
 Setting aside these concerns, a side effect of OCRmyPDF is it may
 incidentally sanitize PDFs that contain certain types of malware. It
-runs ``qpdf`` to repair the PDF, which could correct malformed PDF
+repairs the PDF with pikepdf/libqpdf, which could correct malformed PDF
 structures that are part of an attack. When PDF/A output is selected
 (the default), the input PDF is partially reconstructed by Ghostscript.
 When ``--force-ocr`` is used, all pages are rasterized and reconverted
@@ -144,10 +144,9 @@ set, the document cannot be viewed without the password.
 Either way, OCRmyPDF does not remove passwords from PDFs and exits with
 an error on encountering them.
 
-``qpdf``, one of OCRmyPDF's dependencies, can remove passwords. If the
-owner and user password are set, a password is required for ``qpdf``. If
-only the owner password is set, then the password can be stripped, even
-if one does not have the owner password.
+``qpdf`` can remove passwords. If the owner and user password are set, a
+password is required for ``qpdf``. If only the owner password is set, then the
+password can be stripped, even if one does not have the owner password.
 
 After OCR is applied, password protection is not permitted on PDF/A
 documents but the file can be converted to regular PDF.