Here's a sample BP that calls BO asynchronously. BO returns  current request state, upon which BP decides to wait some more, report an error or process an answer:

Class test.BP Extends Ens.BusinessProcess
{

/// Operation name
Property Operation As %String(MAXLEN = 250) [ Required ];

/// How long to wait for an answer. 0 - forever.
Property MaxProcessTime As %Integer(MINVAL = 0) [ InitialExpression = 3600 ];

Parameter SETTINGS = "MaxProcessTime:Basic,Operation:Basic:selector?context={Ens.ContextSearch/ProductionItems?targets=1&productionName=@productionId}";

/// Identifier for a first request
Parameter callCOMPLETIONKEY = "*call*";

/// Identifier for a state request
Parameter getStateCOMPLETIONKEY = "*getState*";

/// Alarm identifier
Parameter alarmCOMPLETIONKEY = "*alarm*";

Method OnRequest(pRequest As Ens.StringRequest, Output pResponse As Ens.StringResponse) As %Status
{
    #dim msg As Ens.StringRequest = pRequest.%ConstructClone(1)
    quit ..SendRequestAsync(..Operation, msg, $$$YES, ..#callCOMPLETIONKEY)
}

/// Process Async reply from Operation
Method OnResponse(request As Ens.StringRequest, ByRef response As Ens.StringResponse, callrequest As Ens.StringRequest, callresponse As Ens.StringResponse, pCompletionKey As %String) As %Status
{
    #dim sc As %Status = $$$OK
    
    // Got an error
    if $$$ISERR(callresponse.status)
    {
        set response = callresponse
        quit $$$OK
    }
    
    // Got primary answer
    if (pCompletionKey = ..#callCOMPLETIONKEY) || (pCompletionKey = ..#alarmCOMPLETIONKEY)
    {
        quit ..OnResponseFromCallOrAlarm(request, .response, callrequest, callresponse, pCompletionKey)
    }
    
    // Got getState
    if (pCompletionKey = ..#getStateCOMPLETIONKEY)
    {
        
        // Current processing state (1 - received; 2 - in work; 3 - done)
        #dim status As %String = callresponse.StringValue

        // If not 3, run getState again
        if (status '= "3")
        {        
            // Check how much time passed since we started
            set processTime = $system.SQL.DATEDIFF("s", ..%TimeCreated, $$$ts)
            
            if ((..MaxProcessTime=0) || (processTime<..MaxProcessTime)) {
                // Let's run getState again in 30 seconds
                #dim alarmMsg As Ens.AlarmRequest = ##class(Ens.AlarmRequest).%New()
                set alarmMsg.Duration = "PT30S"
                quit ..SendRequestAsync("Ens.Alarm", alarmMsg, $$$YES, ..#alarmCOMPLETIONKEY)
            } else {
                // Timeout
                set response = ##class(Ens.StringResponse).%New()
                set response.status = $$$ERROR($$$GeneralError, "Timeout")
                quit $$$OK    
            }
        }
        else
        {
            quit ..OnResponseFromGetState3(request, .response, callrequest, callresponse, pCompletionKey)
        }
    }
    
    // unrecognized pCompletionKey value
    quit $$$ERROR($$$InvalidArgument)
}

/// OnResponse for alarm or call - run get state
Method OnResponseFromCallOrAlarm(request As Ens.StringRequest, ByRef response As Ens.StringResponse, callrequest As Ens.StringRequest, callresponse As Ens.StringResponse, pCompletionKey As %String) As %Status [ Private ]
{
    #dim msg As Ens.StringRequest = pRequest.%ConstructClone(1)
    quit ..SendRequestAsync(..Operation, msg, $$$YES, ..#getStateCOMPLETIONKEY)
}

/// OnResponse for an answer
Method OnResponseFromGetState3(request As Ens.StringRequest, ByRef response Ens.StringResponse, callrequest As Ens.StringRequest, callresponse As Ens.StringResponse, pCompletionKey As %String) As %Status [ Private ]
{
    // Process complete response
    quit $$$OK
}

OnResponse would be an automatic observer, and can in turn notify anything else. You can use original request id or SessionID to identify what you need to notify.

There is no Cache function to do that.

But as PDF contains readable "postscript" parts you can use regexp to search for relevant information. Stack. Article. It's not guaranteed to be precise though.

Here's my article about using LibreOffice for work with documents. I've also used ghostscript and postscript to work with pdf from Caché and it's all fairly straightforward.

Also, here's the code I wrote (execute is defined here) to add footer to every page of a PDF file using ghostscript:

/// Use ghostscript (%1) to apply postscript script %3
/// Upon source pdf (%4) to get output pgf (%2)
/// Attempts at speed  -dProvideUnicode -dEmbedAllFonts=true  -dPDFSETTINGS=/prepress
Parameter STAMP = "%1  -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=%2 %3 -f %4";

ClassMethod stampPDF(pdf, psFile, pdfOut) As %Status
{
    set cmd = $$$FormatText(..#STAMP, ..getGS(), pdfOut, psFile, pdf)
    return ..execute(cmd)
}

ClassMethod createPS(psFile, text) As %Status
{
    set stream = ##class(%Stream.FileCharacter).%New()
        // For cyrillic text
    set stream.TranslateTable = "CP1251"
    set sc = stream.LinkToFile(psFile)
    quit:$$$ISERR(sc) sc
    
    do stream.WriteLine("<<")
    do stream.WriteLine("   /EndPage")
    do stream.WriteLine("   {")
    do stream.WriteLine("     2 eq { pop false }")
    do stream.WriteLine("     {")
    do stream.WriteLine("         gsave")
    do stream.WriteLine("         /MyFont 12 selectfont")
    do stream.WriteLine("         30 70 moveto (" _ text _ ") show")
    do stream.WriteLine("         grestore")
    do stream.WriteLine("         true")
    do stream.WriteLine("     } ifelse")
    do stream.WriteLine("   } bind")
    do stream.WriteLine(">> setpagedevice")
    
    quit stream.%Save()
}

/// Get gs binary
ClassMethod getGS()
{
    if $$$isWINDOWS {
        set gs = "gswin64c"
    } else {
        set gs = "gs"
    }
    return gs
}

Ghostscript can be used to get a number of pages in a PDF file. Here's how.

In your BO try:

Set result = ..Adapter.FTP.MakeDirectory(.path)

Adapter is EnsLib.FTP.OutboundAdapter and FTP is %Net.FtpSession.

I would subclass the adapter and add MakeDirectory method there, with code pointing to the FTP method and status conversion.