U+FDDF is not a valid Unicode code point / character.
It belongs to the "Arabic Presentation Forms-A" block which goes from 0xFB50 to 0xFDFF but is not defined in the Unicode 15.0 database of code points.
This code point first appeared in version of the Unicode® Standard and belongs to the "Arabic Presentation Forms-A" block which goes from 0xFB50 to 0xFDFF.
You can safely add this character in your html code with the entity: 
You can use the u+FDDF copy pc button below.
Easy u+FDDF copy paste:
Unicode meta-data
The following table show specific meta-data that is known about this character.
| field | value |
|---|---|
| Codepoint (hex) | FDDF, uFDDF |
| Legacy name (Unicode 1.0) | - |
| Official name (Unicode 15.0) | - |
| resolved name | |
| block | Arabic Presentation Forms-A (Arabic_PF_A) |
| common typos | u+DFDF, u+DFDF |
There are alternative spelling that can be found in the wild for the unicode character FDDF like u FDDF, (u+FDDF) or u +FDDF. You can also find u-FDDF, u*FDDF, un+FDDF, uFDDF, u=FDDF or c+FDDF. You can also spell it with u FDDF unicode, u plus FDDF, uncode FDDF or unicode + FDDF.
Glyphs and symbols in your browser
The following unicode chart presents different versions of the glyph corresponding to the unicode characters u+FDDF that are available on your computer.
In order to type this character easily, you may want to download and install a unicode Arabic Presentation Forms-A keyboard.
A sample of fonts are used below to display whether the character has a glyph in this font or not.
Encodings (Unicode characters converter)
The following character table converter for +uFDDF allows you to see the value of the character in different encodings
| encoding | hexadecimal | decimal | binary |
|---|---|---|---|
| UTF-8 | EF B7 9F | 15710111 | 11101111 10110111 10011111 |
| UCS2 | DF FD | 57341 | 11011111 11111101 |
| UTF-16LE | DF FD | 57341 | 11011111 11111101 |
| UTF-16BE | FD DF | 64991 | 11111101 11011111 |
| UTF7 | 2B 2F 64 38 2D | 185478690861 | 00101011 00101111 01100100 00111000 00101101 |
| UTF7-IMAP | 26 2C 64 38 2D | 163953522733 | 00100110 00101100 01100100 00111000 00101101 |