From the Part 4: PAdES Long Term - PAdES-LTV Profile (ETSI TS 102 778-4) Specification: 

"Validation of an electronic signature requires data to validate the signature such as CA certificates, Certificate Revocation List (CRLs) or Certificate status information (OCSP) commonly provided by an online service (referred to in the present document as validation data). If the document is stored and the signatures are to be verifiable long after first created, in particular after the signing certificate has expired, the original validation data may no longer available or there may uncertainty as to what validation data was used when the document was first verified."

Images are decompressed during import/export which result in significantly larger files

 PdfProcesing: Add support for Transparency Group XObjects

In specific cases when signing a document and exporting it the signature is not properly exported and it is not shown in the Signature panel:

Images could have orientation set in their metadata:

Workaround: Rotate the image before inserting it into the RadFixedPage (check the attached project).

The CombTextBoxField text alignment is wrong when a file is imported and then exported.

A text annotation represents a “sticky note” attached to a point in the PDF document. When closed, the annotation appears as an icon; when opened, it displays a pop-up window containing the text of the note in a font and size chosen by the viewer application.

The auto-size fields are not correctly imported/exported this setting is lost during export 

A table that contains a cell with a single block with a lot of new lines is not split correctly.

According to the Pdf Specification, the ExtGState property is optional but in this specific scenario the property is not omitted but just the object implementation.

This case can be handled using the Handling Exceptions mechanism.

By specification, the FontDescriptor is required except for Standard fonts. However, if some font has only a declaration of FontDescriptor but with a missing definition, the document is not exported successfully.

Ordered list marker color not properly imported from inline CSS style in HTML.

With the current implementation when exporting a Stitching function all the containing functions are exported as Sampled functions even if they originally have been imported as Exponential interpolation functions, which leads to an increase in the size of the document.

More information can be found in the PDF Specification.

Key not found exception when there are missing characters in the document. 

Field border appearance are lost during import-export of the document

The table alignment is not maintained when the table is split into multiple pages. 

Inserting a LineBreak in a TableCell Block with text exceeding Cell's width can break rendering the rest of the text, which should go on the next line of the Cell. This can result in the next line of text being invisible. 

Handle the case where the document has edit, copy password but no open password. We should either suppress the UserPasswordNeeded event or provide information about the password options in the arguments. 

According to the PDF Specification: The definition of an indirect object in a PDF file consists of its object number and generation number, followed by the value of the object bracketed between the keywords obj and endobj. For example, the definition:
An invalid case:

When exporting a document with a Type 1 font with custom encoding an exception is thrown: System.NotImplementedException: 'The method or operation is not implemented.'