How can I read out internal pdf creation/modified date with Windows PowerShell?

You can read a PDF file (at least you can in newer formats) as though it were text. You will find an embedded XML section that uses the Adobe XMP schema. This contains the metadata you need.

Here is an example:

%PDF-1.5
%âãÏÓ
2 0 obj
<<
/AcroForm 4 0 R
/Lang (en-GB)
/MarkInfo <<
/Marked true
>>
/Metadata 5 0 R
/Pages 6 0 R
/StructTreeRoot 7 0 R
/Type /Catalog
>>
endobj
5 0 obj
<<
/Length 2971
/Subtype /XML
/Type /Metadata
>>
stream
<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 5.1.2">
    <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
        <rdf:Description rdf:about=""
                xmlns:xmp="http://ns.adobe.com/xap/1.0/">
            <xmp:CreateDate>2014-03-05T15:03:02+01:00</xmp:CreateDate>
            <xmp:ModifyDate>2014-05-30T11:58:02+01:00</xmp:ModifyDate>
            <xmp:MetadataDate>2014-03-05T14:03:46Z</xmp:MetadataDate>
        </rdf:Description>
        <rdf:Description rdf:about=""
                xmlns:xmpMM="http://ns.adobe.com/xap/1.0/mm/">
            <xmpMM:DocumentID>uuid:8b5fe011-ed77-4298-aa84-d1eda797b9ff</xmpMM:DocumentID>
            <xmpMM:InstanceID>uuid:88074e0b-42f7-4268-bc89-0162e417c9ad</xmpMM:InstanceID>
        </rdf:Description>
        <rdf:Description rdf:about=""
                xmlns:dc="http://purl.org/dc/elements/1.1/">
            <dc:format>application/pdf</dc:format>
        </rdf:Description>
    </rdf:RDF>
</x:xmpmeta>

The following example will retrieve the create date:

$a = Select-String "CreateDate\>(.*)\<" .\filename.pdf

Which returns something like:

filename.pdf:20:         <xap:CreateDate>2009-11-03T10:54:29Z</xap:CreateDate>
filename.pdf:12921:         <xap:CreateDate>2009-11-03T10:54:29Z</xap:CreateDate>

Getting to the exact data:

$a.Matches.Groups[1]

Which returns:

2009-11-03T10:54:29Z

First of all, you need to get access to a .net library which is able to read the document properties as these are not native shell properties: http://sourceforge.net/projects/itextsharp/

Next up, you need to look at scripting out the objects from the pdf such as:

# load ITextSHarp.dll

[System.Reflection.Assembly]::LoadFrom("C:\users\testuser\desktop\itextsharp.dll")

$raf = New-object iTextSharp.text.pdf.RandomAccessFileOrArray("C:\users\testuser\desktop\bitcoin.pdf")

# load pdf properties

$reader = New-object iTextSharp.text.pdf.PdfReader($raf, $Nothing)

$reader