/**
 * Represents the custom metadata for a PDF document.
 * Provides methods to manage key-value metadata pairs.
 *
 * ```typescript
 * // Load an existing PDF document
 * let document: PdfDocument = new PdfDocument(data, password);
 * // Access the document properties
 * let documentProperties: PdfDocumentProperites = document.getDocumentInformation(false);
 * // Gets the custom metadata
 * let custom: PdfCustomMetadata = documentProperties.customMetadata;
 * // Sets custom value
 * custom.set('key','value');
 * // Sets the document Information
 * document.setDocumentInformation(documentProperties);
 * // Saves the document
 * document.save();
 * ```
 */
export declare class PdfCustomMetadata {
    /**
     * Stores custom metadata properties.
     *
     * @private
     *
     */
    _customData: Map<string, string>;
    private _standardKeys;
    /**
     * Sets a key to the custom data map.
     *
     * @param {string} key The metadata key.
     * @param {string} value The metadata value.
     * @returns {void} Nothing
     *
     * ```typescript
     * // Load an existing PDF document
     * let document: PdfDocument = new PdfDocument(data, password);
     * // Access the document informations
     * let documentProperties: PdfDocumentProperites = document.getDocumentInformation(false);
     * // Gets the custom metadata from document information
     * let custom: PdfCustomMetadata= documentProperties.customMetadata;
     * // Sets a custom metadata value
     * custom.set('key','value');
     * // Sets the document Information
     * document.setDocumentInformation(documentProperties);
     * // Saves the document
     * document.save();
     * ```
     */
    set(key: string, value: string): void;
    /**
     * Gets a key from the custom data map.
     *
     * @param {string} key The key to get.
     * @returns {string} value The value of the key.
     *
     * ```typescript
     * // Load an existing PDF document
     * let document: PdfDocument = new PdfDocument(data, password);
     * // Access the document informations
     * let documentProperties: PdfDocumentProperites = document.getDocumentInformation(false);
     * // Gets the custom metadata from document information
     * let custom:PdfCustomMetadata = documentProperties.customMetadata;
     * // Gets a custom metadata value
     * let value: string = custom.get('key');
     * // Saves the document
     * document.save();
     * ```
     */
    get(key: string): string;
    /**
     * Checks whether a key exists in the custom data map.
     *
     * @param {string} key The key to check.
     * @returns {boolean} True if the key exists; otherwise false.
     *
     * ```typescript
     * // Load an existing PDF document
     * let document: PdfDocument = new PdfDocument(data, password);
     * // Access the document informations
     * let documentProperties: PdfDocumentProperites = document.getDocumentInformation(false);
     * // Gets the custom metadata from document information
     * let custom:PdfCustomMetadata = documentProperties.customMetadata;
     * // Check the key is present in the custom metadata
     * let exists: boolean = custom.has('key');
     * // Saves the document
     * document.save();
     * ```
     */
    has(key: string): boolean;
    /**
     * Removes a key from the custom data map.
     *
     * @param {string} key The key to remove.
     * @returns {void} Nothing.
     *
     * ```typescript
     * // Load an existing PDF document
     * // Access the document informations
     * let documentProperties: PdfDocumentProperites = document.getDocumentInformation(false);
     * // Gets the custom metadata from document information
     * let custom:PdfCustomMetadata = documentProperties.customMetadata;
     * // Removes the key from the custom metadata
     * custom.remove('key');
     * // Sets the document Information
     * document.setDocumentInformation(documentProperties);
     * // Saves the document
     * document.save();
     * ```
     */
    remove(key: string): void;
}