Make sure to have ForceDownload as true and disable the Range Retrival using below setting if needed.
<mediaType name="PDF file" extensions="pdf">
<mimeType>application/pdf</mimeType>
<forceDownload>true</forceDownload>
</mediaType>
<
setting
name
=
"Media.EnableRangeRetrievalRequest"
value
=
"false"
/>