Add pdflayers script

Rough initial version of script. Only tested with
a few PDF files. There's a lot of things that could
be improved:
* Add options to list layer names
* Add options for trying to loacate and modify JavaScript
  that might affect layer visibility (in Adobe Reader).
* Error handling...
* Cleanup... better ways of using pikedpdf..?

pdflayers.py

@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+"""Script for editing PDF to change layer (OCG) visibility.
+
+The output should display correctly in the PDF viwer in google-chrome,
+evince, okular, and Adobe Reader (with JavaScript disabled).
+
+pdf.js in firefox might not work: https://github.com/mozilla/pdf.js/issues/4841
+
+Requirements:
+Install pikepdf:
+  pip3 install --user pikepdf
+and run the script with python3.
+
+References:
+  https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdf_reference_archive/PDFReference16.pdf
+  https://pikepdf.readthedocs.io/
+"""
+import argparse
+import logging
+import sys
+
+import pikepdf
+
+
+def set_layer_visibility(pdf, layers_to_show):
+    """Set visibility of layers."""
+    try:
+        ocgs = pdf.root.OCProperties.OCGs
+    except (AttributeError, KeyError):
+        logging.error("Unable to locate layers in PDF.")
+        sys.exit(1)
+
+    ocgs_on = []
+    ocgs_off = []
+    for ocg in ocgs:
+        if ocg.Name in layers_to_show:
+            logging.info("Layer %s will be visible.", ocg.Name)
+            ocgs_on.append(ocg)
+        else:
+            logging.info("Layer %s will be hidden.", ocg.Name)
+            ocgs_off.append(ocg)
+
+    ocgs_config = pikepdf.Dictionary(
+        BaseState=pikepdf.Name('/OFF'),
+        ON=ocgs_on,
+    )
+
+    pdf.root.OCProperties = pikepdf.Dictionary(
+        D=ocgs_config,
+        OCGs=ocgs,
+    )
+
+    # Needed for the PDF viwer in google-chrome (at least):
+    # TODO: Set PrintState and ExportState too..? Exception+ handling...
+    # Check PDF reference. Maybe better to just delete ViewState..?
+    for ocg in ocgs_on:
+        ocg.Usage.View.ViewState = pikepdf.Name('/ON')
+    for ocg in ocgs_off:
+        ocg.Usage.View.ViewState = pikepdf.Name('/OFF')
+
+
+def main():
+    """Main."""
+    logging.basicConfig(level=logging.INFO)
+    parser = argparse.ArgumentParser(
+        description='Tool for changing default visibility of PDF layers.'
+    )
+    parser.add_argument('input',
+                        help='input file')
+    parser.add_argument('output',
+                        help='output file')
+    parser.add_argument('--password', default='',
+                        help='user password (for encrypted files)')
+    parser.add_argument('--show', nargs='+', default=(),
+                        help='layers to be visible in output')
+    parser.add_argument('--qdf', action='store_true',
+                        help='Use qdf to save the output. (Debug option)')
+    args = parser.parse_args()
+
+    pdf = pikepdf.open(args.input, password=args.password)
+
+    set_layer_visibility(pdf, args.show)
+    pdf.remove_unreferenced_resources()
+
+    save_options = {
+        'compress_streams': False,
+        'qdf': True,
+    } if args.qdf else {
+    }
+
+    pdf.save(args.output, **save_options)
+
+
+if __name__ == "__main__":
+    main()