FPDF_Creator can only create file through embedder-supplied callback.

core/src/fxcrt/fx_basic_buffer.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "../../include/fxcrt/fx_basic.h"
 FX_STRSIZE FX_ftoa(FX_FLOAT f, FX_CHAR* buf);
 CFX_BinaryBuf::CFX_BinaryBuf()
     : m_AllocStep(0), m_pBuffer(NULL), m_DataSize(0), m_AllocSize(0) {}
@@ skipping 309 matching lines @@
   if (!pFile) {
     return FALSE;
   }
   if (m_pFile && m_bTakeover) {
     m_pFile->Release();
   }
   m_pFile = pFile;
   m_bTakeover = bTakeover;
   return TRUE;
 }
-FX_BOOL CFX_FileBufferArchive::AttachFile(const FX_WCHAR* filename) {
-  if (!filename) {
-    return FALSE;
-  }
-  if (m_pFile && m_bTakeover) {
-    m_pFile->Release();
-  }
-  m_pFile = FX_CreateFileWrite(filename);
-  if (!m_pFile) {
-    return FALSE;
-  }
-  m_bTakeover = TRUE;
-  return TRUE;
-}
-FX_BOOL CFX_FileBufferArchive::AttachFile(const FX_CHAR* filename) {
-  if (!filename) {
-    return FALSE;
-  }
-  if (m_pFile && m_bTakeover) {
-    m_pFile->Release();
-  }
-  m_pFile = FX_CreateFileWrite(filename);
-  if (!m_pFile) {
-    return FALSE;
-  }
-  m_bTakeover = TRUE;
-  return TRUE;
-}
 FX_BOOL CFX_FileBufferArchive::DoWork(const void* pBuf, size_t size) {
   if (!m_pFile) {
     return FALSE;
   }
   if (!pBuf || size < 1) {
     return TRUE;
   }
   return m_pFile->WriteBlock(pBuf, size);
 }