Default render mode renders a PDF document using native XAML elements with UI virtualization. This "true XAML" approach allows some special features not available in System render mode, like text select and search. If you need these features, as well as other features like bookmark navigation and annotations, then you should use Default render mode.

The full list of limitations can be found below:

Fonts

PdfViewer for UWP supports the following font types:

• Embedded TrueType fonts: These are fonts specified using the "FontFile2" mechanism in PDF.
• OpenType fonts: These are successors to TrueType fonts and provide cross-platform compatibility as well as extensive language support.
• Silverlight fonts: This includes all font families supported by Silverlight.
• PDF base fonts: This includes fonts built into Adobe Acrobat such as Helvetica, Times, and Symbol.

The C1PdfViewer control does not support other font types available in the PDF specification, including Adobe Type 1 fonts (specified using the "FontFile" mechanism in the PDF file).

Documents that use non-supported fonts will still render, but the formatting will be incorrect (for example, the document may show overlapping text).

Images

PdfViewer for UWP supports most common image types, including all binary stream formats supported by Silverlight as well as deflated streams of several types (RGB, Monochrome, and several common indexed formats).
The C1PdfViewer control does not support some rare formats such as deflated JPG streams, or advanced features such as custom color spaces or halftones. C1PdfViewer does not support DCTDecode/JPEG image encoding. Note that scanned PDF files may contain TIFF data which the C1PdfViewer control is currently not capable of rendering.

Masks

The C1PdfViewer control does not support soft masks (specified using the "SMask" mechanism in the PDF file).