Attachment objPDF = [SELECT ID, Name, Body FROM Attachment where id = '00P0I00000u85YXUAY'
    LIMIT 1
];
String strTargetURL = 'https://www.perfectaudit.com/api/v1/book/upload';

String boundary = '----WebKitFormBoundary7MA4YWxkTrZu0gW';
string header1 = '--' + boundary + 'nContent-Disposition: form-data; name="pk"';
String header1Encoded = EncodingUtil.base64Encode(Blob.valueOf(header1 + 'rnrn'));
while (header1Encoded.endsWith('=')) {
    header1 += ' ';
    header1Encoded = EncodingUtil.base64Encode(Blob.valueOf(header1 + 'rnrn'));
}
String body1Encoded = EncodingUtil.base64Encode(Blob.valueOf('68749rn'));
String last4Bytes = body1Encoded.substring(body1Encoded.length() - 4, body1Encoded.length());
if (last4Bytes.endsWith('==')) {
    last4Bytes = last4Bytes.substring(0, 2) + '0K';
    body1Encoded = body1Encoded.substring(0, body1Encoded.length() - 4) + last4Bytes;
} else if (last4Bytes.endsWith('=')) {
    last4Bytes = last4Bytes.substring(0, 3) + 'N';
    body1Encoded = body1Encoded.substring(0, body1Encoded.length() - 4) + last4Bytes;
}

String header = '--' + boundary + 'nContent-Disposition: form-data; name="upload"; filename="' + objPDF.Name + '";nContent-Type: application/pdf';
String footer = '--' + boundary + '--';
String headerEncoded = EncodingUtil.base64Encode(Blob.valueOf(header + 'rnrn'));
while (headerEncoded.endsWith('=')) {
    header += ' ';
    headerEncoded = EncodingUtil.base64Encode(Blob.valueOf(header + 'rnrn'));
}
String bodyEncoded = EncodingUtil.base64Encode(Blob.valueOf('This is text file'));
Blob bodyBlob = null;
last4Bytes = bodyEncoded.substring(bodyEncoded.length() - 4, bodyEncoded.length());
if (last4Bytes.endsWith('==')) {
    last4Bytes = last4Bytes.substring(0, 2) + '0K';
    bodyEncoded = bodyEncoded.substring(0, bodyEncoded.length() - 4) + last4Bytes;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(header1Encoded + body1Encoded + headerEncoded + bodyEncoded + footerEncoded);
} else if (last4Bytes.endsWith('=')) {
    last4Bytes = last4Bytes.substring(0, 3) + 'N';
    bodyEncoded = bodyEncoded.substring(0, bodyEncoded.length() - 4) + last4Bytes;
    footer = 'rn' + footer;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(header1Encoded + body1Encoded + headerEncoded + bodyEncoded + footerEncoded);
} else {
    footer = 'n' + footer;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(header1Encoded + body1Encoded + headerEncoded + bodyEncoded + footerEncoded);
}

HttpRequest req = new HttpRequest();

String userName = 'xxxxx';
String password = 'xxxxx';
Blob headerValue = Blob.valueOf(userName + ':' + password);
String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
req.setHeader('Authorization', authorizationHeader);

req.setHeader('Content-Type', 'multipart/form-data; boundary=' + boundary);
req.setMethod('POST');
req.setEndpoint(strTargetURL);
req.setBodyAsBlob(bodyBlob);
req.setTimeout(120000);

Http http = new Http();
HTTPResponse res = http.send(req);
system.debug(res.getBody());