Does not work with dynamic object class properties unfortunately:

Class User.DO Extends %Persistent
{

Property MyProp As %DynamicObject;

/// do ##class(User.DO).Test()
ClassMethod Test()
{
    do ..%KillExtent()
    // 11 = $length({"prop":""}.%ToJSON())
    for len = 100, $$$MaxStringLength - 11, $$$MaxStringLength - 11 + 1 {
        set sc = ..Create(len)
        write "len: ", len, " result: ", $case($$$ISOK(sc), $$$YES: "OK", : "ERROR: " _ $system.Status.GetErrorText(sc)), !
        quit:$$$ISERR(sc)
    }
}

ClassMethod Create(len) As %Status
{
    set obj = ..%New()
    do obj.MyProp.%Set("prop", ..GetStream(len), "stream")
    set sc = obj.%Save()
    quit sc
}

ClassMethod GetStream(len) As %Stream.TmpCharacter
{
    set chunk = 1000000
    set stream = ##class(%Stream.TmpCharacter).%New()
    for i=1:chunk:len-chunk {
        do stream.Write($tr($j("", chunk)," ", 0))		
    }
    do stream.Write($tr($j("", len#chunk)," ", 0))
    quit stream
}

}

Results in:

len: 100 result: OK
len: 3641133 result: OK
len: 3641134 result: ERROR: ERROR #5002: ObjectScript error: <MAXSTRING>%GetSerial+1^%Library.DynamicAbstractObject.1 [%GetSerial+1^%Library.DynamicAbstractObject.1:XF]

Same issue if there are several short properties in dynamic object with total length > 3641144 characters.

We need something like:

Property MyProp As %DynamicObject(STORAGE="stream");

If I define %DynamicObject property and its serialization is longer than 3641144 characters, would that work?

Yes, well, you explicitly set your error in:

do {
$$$ASSERT(0) // Subclass Responsibility
Set tSC = $$$EnsError($$$EnsErrNotImplemented,$$$CurrentClass,$$$CurrentMethod)
} while (0)
Exit
Quit tSC
Trap
Set $ZT="",tSC=$$$EnsSystemError
Goto Exit
} 

You need to remove that.

Also Patient info should be in SDA already, so you can remove:

Set target.Patient.Name=source.GetValueAt("PID:5")
Set target.Patient.BirthGender=source.GetValueAt("PID:8")

Code block in my answer is exactly that. What error are you getting  with it?

Both can hold 3641144 at the most.

%String as defined by OP (without MAXLEN) only holds 50 characters, %VarString holds 3641144.

I moved the package to a new name 

Why?

Check the docs from @Marc Mundt, specifically Binary Contains.

import math
math.pi
>3.141592653589793

You can generate methods using Native API for Python, so technically yes.

I'd suggest using source control and CI/CD to track changes.

Additionally you can add parent commit hash and compile time automatically.

1. I have a PDF file which I need to read  from a folder location as text and put data from PDF into HL7 message and send it to downstream system.

Do you mean OCR/text layer extraction?

2. I have a PDF file which I need to read  from a folder location encode it in base64 and put in OBX.5  of MDM message

Do it like this.

Have you tried executing the same code from the InterSystems terminal?

Also try:

text = self.db.run_class_method("%SYSTEM.Status", "GetErrorText", res)
print(text)

Yes, everything $isobject is a reference.

I know how to shave 3 symbols from that...