Fix build error on win x64.

samples/pdfium_test.cc

diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 76862be..0c64149 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -180,7 +180,7 @@
 
   FPDF_FILEACCESS file_access;
   memset(&file_access, '\0', sizeof(file_access));
-  file_access.m_FileLen = len;
+  file_access.m_FileLen = static_cast<unsigned long>(len);
   file_access.m_GetBlock = Get_Block;
   file_access.m_Param = &loader;