Encryption Algorithm 4 is implemented for R1 2018 official release. This includes all encryptions except AES 256 which is related to Encryption Algorithm 5. For AES 256 you may vote and follow this feedback item:
https://feedback.telerik.com/Project/184/Feedback/Details/240154-pdfprocessing-support-encryption-algorithm-5-with-aes-256

This would ensure preserving the existing PDF file pages and document properties untouched and additionally would allow making a few modifications to the file. By appending bytes to existing PDF file one may achieve:

 - Modifying specific pages content and properties.
 - Adding new pages to pages collection.
 - Modify some document properties (such as Bookmarks).
 - Edit interactive forms field values.
 - Add digital signatures to existing documents.

This type of destinations is not supported and leads to InvalidCastException when importing such elements.

Currently, there is no way to know when a new page is created when content "overflows" to the next page 

This event may be needed in order to generate the header and footer based on the currently generated content. In simple header/footer scenarios, however, this event may not be needed and the headers and footers may be drawn after all pages are generated as described in the comments below.

InvalidOperationException is thrown when exporting font that is available but not used.

The stack trace is as follows:
   at System.Linq.Enumerable.Max(IEnumerable`1 source)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.Cid.CidSet.CopyPropertiesFrom(IPdfExportContext context, FontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontDescriptor.CalculateCidSet(IPdfExportContext context, CidFontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontDescriptor.<>c__DisplayClass34.<CopyPropertiesFrom>b__30()
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Utilities.PdfObjectsExtensions.ToPrimitive[P,T](PdfProperty`1 pdfProperty, Func`2 convertToPrimitive, Func`1 getDefaultValue)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontDescriptor.CopyPropertiesFrom(IPdfExportContext context, FontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontObject.CopyPropertiesFromOverride(IPdfExportContext context, FontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontObject.CopyPropertiesFrom(IPdfExportContext context, FontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.Type0FontObject.CopyPropertiesFromOverride(IPdfExportContext context, FontBase font)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Export.PdfExporter.WriteFontsFromContext(PdfWriter writer, IPdfExportContext context)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Export.PdfExporter.Export(IRadFixedDocumentExportContext context, Stream output)
   at Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider.ExportOverride(RadFixedDocument document, Stream output)
   at Telerik.Windows.Documents.Common.FormatProviders.FormatProviderBase`1.Export(T document, Stream output)

The exception is thrown with the message "Password is not correct" even when the user password is correct. This issue occurs for specific encryption algorithm parameters only.

This includes loading form fields data from FDF file and saving the form fields data to FDF file.

This exception seems to be related to AES algorithm PaddingMode property. 

The signature field preserves information about the time of signing through the TimeOfSigning property of Signature.Properties. However, the DateTime object doesn't keep info about the time zone the document has been signed in. Some customers need to obtain this information.

While signing a document in Adobe Reader, the users can lock the document fields from editing. Expose a similar functionality.

The feature is described by the FieldMDP transform method, described on page 736 from the Pdf Reference 1.7

The case is invalid according to the specification but all readers manage to open the document and allow editing its fields. The document can be also opened in RadPdfViewer but without the fields and the widgets.

RadPdfProcessing cannot find startxref keyword at the end of a document if the document contains many zero bytes at its end. NotSupportedException is thrown with "Startxref keyword cannot be found" message. 
Workaround: Trim the ending zero bytes before loading the document to PdfProcessing. Example showing this approach can be found in the following KB article:
http://www.telerik.com/support/kb/winforms/details/notsupportedexception-startxref-keyword-cannot-be-found-when-loading-pdf-file-in-radpdfviewer-for-winforms

Documents can be signed using the end certificate of a certificate chain, instead of the whole chain. This improves the performance of the signing.

When merging files the documents might have the same fonts embedded. Then the PdfProcessing should use only one of the fonts and skip embedding the second font in the resulting document.

Add support for Lab color space to allow clients to import and export documents with Lab color space.

When a document is imported the FontsRepository caches the created font instances for performance reasons. API should be provided for clearing the cached instances in order to reduce memory consumption.

The values of rotated widgets on a rotated page are invisible after exporting them. The value can be seen only while editing a field.

The calculations are wrong, leading to single lines split by the LineSpacing set. As a result, the content of the PDF document is laid out on a bigger number of pages.

Workaround: Change the LineSpacingType to HeightType.Auto.

The checkbox state is not correctly imported when the value is not specified

By specification, the URI property of URI Actions is encoded in 7-bit ASCII string. For this reason, our current implementation relies on that the property will be of type PdfLiteralString. However, in cases such as password-encrypted documents, the property may be encoded with PdfHexString which leads to InvalidCastException: 'Unable to cast object of type Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfHexString' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Types.PdfLiteralString'.', on document import.