You should DEFINITELY have a look at PyPDF2. This amazing library has incredible potential, you can extract whatever from a PDF, including images or comments. Try to start by examining what Acrobat Reader DC (Reader) can give you on a PDF’s comments. Take a simple PDF, annotate it (add some comments) with Reader and in the comments tab in the upper right corner, click the horizontal three dots and click Export All To Data File... and select the format with the extension xfdf. This creates a wonderful xml file which you can parse. The format is very transparent and self-evident.
If, however, you cannot rely on a user clicking this and instead need to extract the same data from a PDF programmatically using python, do not despair, there is a solution. (Inspired by Extract images from PDF without resampling, in python?)
Prerequisites
pip install PyPDF2
xfdf XML
What Reader gives you in the above mentioned xfdf file, looks like this:
<?xml version="1.0" ?>
<xfdf xml:space="preserve" xmlns="http://ns.adobe.com/xfdf/">
<annots>
<caret IT="Replace" color="#0000FF" creationdate="D:20190221151519+01'00'" date="D:20190221151526+01'00'" flags="print" fringe="1.069520,1.069520,1.069520,1.069520" name="72f8d1b7-d878-4281-bd33-3a6fb4578673" page="0" rect="636.942000,476.891000,652.693000,489.725000" subject="Inserted Text" title="Admin">
<contents-richtext>
<body xfa:APIVersion="Acrobat:19.10.0" xfa:spec="2.0.2" xmlns="http://www.w3.org/1999/xhtml" xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/">
<p dir="ltr">
<span dir="ltr" style="font-size:10.5pt;text-align:left;color:#000000;font-weight:normal;font-style:normal"> comment1</span>
</p>
</body>
</contents-richtext>
<popup flags="print,nozoom,norotate" open="no" page="0" rect="737.008000,374.656000,941.008000,488.656000"/>
</caret>
<highlight color="#FFD100" coords="183.867000,402.332000,220.968000,402.332000,183.867000,387.587000,220.968000,387.587000" creationdate="D:20190221151441+01'00'" date="D:20190221151448+01'00'" flags="print" name="a18c7fb0-0af3-435e-8c32-1af2af3c46ea" opacity="0.399994" page="0" rect="179.930000,387.126000,224.904000,402.793000" subject="Highlight" title="Admin">
<contents-richtext>
<body xfa:APIVersion="Acrobat:19.10.0" xfa:spec="2.0.2" xmlns="http://www.w3.org/1999/xhtml" xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/">
<p dir="ltr">
<span dir="ltr" style="font-size:10.5pt;text-align:left;color:#000000;font-weight:normal;font-style:normal">comment2</span>
</p>
</body>
</contents-richtext>
<popup flags="print,nozoom,norotate" open="no" page="0" rect="737.008000,288.332000,941.008000,402.332000"/>
</highlight>
<caret color="#0000FF" creationdate="D:20190221151452+01'00'" date="D:20190221151452+01'00'" flags="print" fringe="0.828156,0.828156,0.828156,0.828156" name="6bf0226e-a3fb-49bf-bc89-05bb671e1627" page="0" rect="285.877000,372.978000,298.073000,382.916000" subject="Inserted Text" title="Admin">
<popup flags="print,nozoom,norotate" open="no" page="0" rect="737.008000,268.088000,941.008000,382.088000"/>
</caret>
<strikeout IT="StrikeOutTextEdit" color="#0000FF" coords="588.088000,497.406000,644.818000,497.406000,588.088000,477.960000,644.818000,477.960000" creationdate="D:20190221151519+01'00'" date="D:20190221151519+01'00'" flags="print" inreplyto="72f8d1b7-d878-4281-bd33-3a6fb4578673" name="6686b852-3924-4252-af21-c1b10390841f" page="0" rect="582.290000,476.745000,650.616000,498.621000" replyType="group" subject="Cross-Out" title="Admin">
<popup flags="print,nozoom,norotate" open="no" page="0" rect="737.008000,383.406000,941.008000,497.406000"/>
</strikeout>
</annots>
<f href="p1.pdf"/>
<ids modified="ABB10FA107DAAA47822FB5D311112349" original="474F087D87E7E544F6DEB9E0A93ADFB2"/>
</xfdf>
Various types of comments are presented here as tags within an <annots> block.
Using PyPDF2
Python can give you almost the same data. To obtain it, have a look at what the output of the following script gives:
from PyPDF2 import PdfFileReader
reader = PdfFileReader("/path/to/my/file.pdf")
for page in reader.pages:
try :
for annot in page["/Annots"] :
print (annot.getObject()) # (1)
print ("")
except :
# there are no annotations on this page
pass
The output for the same file as in the xfdf file above will look like this:
{'/Popup': IndirectObject(192, 0), '/M': u"D:20190221151448+01'00'", '/CreationDate': u"D:20190221151441+01'00'", '/NM': u'a18c7fb0-0af3-435e-8c32-1af2af3c46ea', '/F': 4, '/C': [1, 0.81961, 0], '/Rect': [179.93, 387.126, 224.904, 402.793], '/Type': '/Annot', '/T': u'Admin', '/RC': u'<?xml version="1.0"?><body xmlns="http://www.w3.org/1999/xhtml" xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/" xfa:APIVersion="Acrobat:19.10.0" xfa:spec="2.0.2" ><p dir="ltr"><span dir="ltr" style="font-size:10.5pt;text-align:left;color:#000000;font-weight:normal;font-style:normal">comment2</span></p></body>', '/P': IndirectObject(5, 0), '/Contents': u'otrasneho', '/QuadPoints': [183.867, 402.332, 220.968, 402.332, 183.867, 387.587, 220.968, 387.587], '/Subj': u'Highlight', '/CA': 0.39999, '/AP': {'/N': IndirectObject(202, 0)}, '/Subtype': '/Highlight'}
{'/Parent': IndirectObject(191, 0), '/Rect': [737.008, 288.332, 941.008, 402.332], '/Type': '/Annot', '/F': 28, '/Open': <PyPDF2.generic.BooleanObject object at 0x02A425D0>, '/Subtype': '/Popup'}
{'/Popup': IndirectObject(194, 0), '/M': u"D:20190221151452+01'00'", '/CreationDate': u"D:20190221151452+01'00'", '/NM': u'6bf0226e-a3fb-49bf-bc89-05bb671e1627', '/F': 4, '/C': [0, 0, 1], '/Subj': u'Inserted Text', '/Rect': [285.877, 372.978, 298.073, 382.916], '/Type': '/Annot', '/P': IndirectObject(5, 0), '/AP': {'/N': IndirectObject(201, 0)}, '/RD': [0.82816, 0.82816, 0.82816, 0.82816], '/T': u'Admin', '/Subtype': '/Caret'}
{'/Parent': IndirectObject(193, 0), '/Rect': [737.008, 268.088, 941.008, 382.088], '/Type': '/Annot', '/F': 28, '/Open': <PyPDF2.generic.BooleanObject object at 0x02A42830>, '/Subtype': '/Popup'}
{'/Popup': IndirectObject(196, 0), '/M': u"D:20190221151519+01'00'", '/CreationDate': u"D:20190221151519+01'00'", '/NM': u'6686b852-3924-4252-af21-c1b10390841f', '/F': 4, '/IRT': IndirectObject(197, 0), '/C': [0, 0, 1], '/Rect': [582.29, 476.745, 650.616, 498.621], '/Type': '/Annot', '/T': u'Admin', '/P': IndirectObject(5, 0), '/QuadPoints': [588.088, 497.406, 644.818, 497.406, 588.088, 477.96, 644.818, 477.96], '/Subj': u'Cross-Out', '/IT': '/StrikeOutTextEdit', '/AP': {'/N': IndirectObject(200, 0)}, '/RT': '/Group', '/Subtype': '/StrikeOut'}
{'/Parent': IndirectObject(195, 0), '/Rect': [737.008, 383.406, 941.008, 497.406], '/Type': '/Annot', '/F': 28, '/Open': <PyPDF2.generic.BooleanObject object at 0x02A42AF0>, '/Subtype': '/Popup'}
{'/Popup': IndirectObject(198, 0), '/M': u"D:20190221151526+01'00'", '/CreationDate': u"D:20190221151519+01'00'", '/NM': u'72f8d1b7-d878-4281-bd33-3a6fb4578673', '/F': 4, '/C': [0, 0, 1], '/Rect': [636.942, 476.891, 652.693, 489.725], '/Type': '/Annot', '/RD': [1.06952, 1.06952, 1.06952, 1.06952], '/T': u'Admin', '/RC': u'<?xml version="1.0"?><body xmlns="http://www.w3.org/1999/xhtml" xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/" xfa:APIVersion="Acrobat:19.10.0" xfa:spec="2.0.2" ><p dir="ltr"><span dir="ltr" style="font-size:10.5pt;text-align:left;color:#000000;font-weight:normal;font-style:normal">comment1</span></p></body>', '/P': IndirectObject(5, 0), '/Contents': u' pica', '/Subj': u'Inserted Text', '/IT': '/Replace', '/AP': {'/N': IndirectObject(212, 0)}, '/Subtype': '/Caret'}
{'/Parent': IndirectObject(197, 0), '/Rect': [737.008, 374.656, 941.008, 488.656], '/Type': '/Annot', '/F': 28, '/Open': <PyPDF2.generic.BooleanObject object at 0x02A42AB0>, '/Subtype': '/Popup'}
If you examine the output, you will realize that the outputs are all more or less the same. Every comment in the xfdf file has two counterparts in PyPDF2’s output in python. The /C attribute is the color of the highlight, in RGB, scaled to floats in the range <0, 1>. /Rect defines the bounding box of the comment on the page/spread, in points (1/72 of an inch) relative to the lower-left corner of the page, increasing values going right and up. /M and /CreationDate are modified and creation times, /QuadPoints is an array of [x1, y1, x2, y2, ..., xn, yn] coordinates of a line around the comment, /Subject, /Type, /SubType, /IT identify the type of the comment, /T is probably the creator, /RC is an xhtml representation of the comment’s text if there is one. If there is an ink-drawn comment, it will be presented here as having an attribute /InkList with data in the form [[L1x1, L1y1, L1x2, L1y2, ..., L1xn, L1yn], [L2x1, L2y1, ..., L2xn, L2yn], ..., [Lmx1, Lmy1, ..., Lmxn, Lmyn]] for line 1, line 2, ..., line m.
For a more thorough explanation of the various fields you get from getObject() in the given python code labeled as line (1), please consult https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf and especially the section 12.5 Annotations starting at pages 381–413.
