You need to explicitly set the correct MIME type when embedding the file into the PDF. This is especially important for standards like PDF/A-3 and Factur-X, which require strict metadata and MIME type declarations for embedded files.
The default value /application/octet-stream is too generic and not compliant for XML attachments.
Generate a sample document with RadPdfProcessing and export it with AES-256 encryption. Try to open it on an iPhone with the default Pdf Viewer of iOS. The document is empty. However, the mobile version of Adobe for iOS opens the document successfully.
System.ArgumentException: 'An item with the same key has already been added. Key: Telerik.Windows.Documents.Fixed.Model.InteractiveForms.RadioButtonField'
It is reproducible when you have at least 2 pages with RadioButtonFields. The import operation goes smoothly. However, merging or removing a page after import leads to the described error.
The issue may be reproduced by opening the attached PDF in Adobe Reader. Although the viewer initially shows the characters correctly, when you start typing in the TextBox, the umlaut/diacritics characters get corrupted. In other PDF viewers, the umlaut characters are handled correctly, so the issue seems to related to concrete Adobe Reader encoding handling implementation. The diacritic characters are handled incorrectly in other viewers as well (e.g. Chrome).
When a Pdf document contains TextBoxFields some of which are hidden and you merge it with another document, the hidden state is reset and the field appears in the merged document:
Before:
After: