[SalesForce] How to send attachment in email using ampscript

This definitely possible. You would use a conditional statement as described in the documentation

%%[IF expression1 <comparison operator> expression2 THEN]%%
    [wrapped script or email content]
%%[ELSEIF expression1 <comparison operator> expression3 THEN]%%
    [wrapped script or email content]
%%[ELSE]%%
    [wrapped script or email content]
%%[ENDIF]%%

In your particular case you will also need a DataExtension AMPScript function as well.

Lookup("DEAllMembers","email_address","email_address", emailaddr)
Lookup("DESpecialMembers","email_address","email_address", emailaddr)

Your final AMP script would look something like this -

%%[
    SET @all    Lookup("DEAllMembers","email_address","email_address", emailaddr)
    SET @special    Lookup("DESpecialMembers","email_address","email_address", emailaddr)

    IF EMPTY(@all) AND EMPTY(@special) THEN
]%%

<p>Your content here</p>

%%[ENDIF]%%

A profile attribute could be used to hold the value for the location of the unique location for that subscribers PDF. This profile attribute would need to be created in the UI in the Profile Management section. Lets assume the field is called PDFLocation. The body of the email would have to contain a reference to the AttachFile function referencing that attribute:

%%=Attachfile("HTTP", PDFLocation)=%%

Then in the JSON body for ContactAttributes, pass the value:

{
    "From": {
        "Address": "code@exacttarget.com",
        "Name": "Code@"
    },
    "To": {
        "Address": "example@example.com",
        "SubscriberKey": "example@example.com",
        "ContactAttributes": {
            "SubscriberAttributes": {
                "PDFLocation": "http://www.exampledomain.com/somefile.pdf"
            }
        }
    }
}