mention preserveUnmodifiedOnSave in the PDF rasterizer FAQ

Author: tony-xlh

_articles/faq/when-is-pdf-rasterizer-needed.md

@@ -16,4 +16,6 @@ last_modified: 2025-01-15 14:58:40 +0800
 
 Third-party generated PDF files may house multiple images, text, or annotations in a single PDF page. As these elements must be rendered within the Dynamic Web TWAIN, it will utilize the PDF rasterizer addon.
 
-When importing a PDF file generated by Dynamic Web TWAIN, or if each page of a third-party PDF holds nothing but a single, unadulterated image, there's no need for the PDF rasterizer addon.
+When importing a PDF file generated by Dynamic Web TWAIN, or if each page of a third-party PDF holds nothing but a single image, there's no need for the PDF rasterizer addon.
+
+If you need to append images to an existing PDF and want to keep the original pages instead of rasterized images in the saved PDF, you need to enable the [`preserveUnmodifiedOnSave`](/_articles/info/api/interfaces.md#:~:text=preserveUnmodifiedOnSave){:target="_blank"} property.