How can I add a link annotation to a PDF file?

To add a link annotation to a PDF file, you should first call the FSPDFPage::addAnnot to add an annotation to a specified page, then call FSAction::Create to create an action, and set the action to the added link annotation. Following is the sample code for adding a URI link annotation to the first page of a PDF file:

#define DOCUMENT_PATH [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0]
...
// Get the path of a PDF
NSString* pdfPath = [[NSBundle mainBundle] pathForResource:@"Sample" ofType:@"pdf"];
    
// Initialize a PDFDoc object with the path to the PDF file.
FSPDFDoc *document = [[FSPDFDoc alloc] initWithPath: pdfPath];
    
// load the unencrypted document content.
 [document load:nil];
    
// Get the first page of the PDF file.
FSPDFPage *page = [document getPage:0];
    
// Add a link annotation to the first page.
FSRectF *rect = [[FSRectF alloc] initWithLeft1:250 bottom1:650 right1:450 top1:750];
FSLink *linkAnnot = [[FSLink alloc] initWithAnnot:[page addAnnot: FSAnnotLink rect:rect]];
    
// Create a URI action and set the URI.
FSURIAction *uriAction = [[FSURIAction alloc] initWithAction:[FSAction create:document action_type:FSActionTypeURI]];
[uriAction setURI:@"https://www.foxit.com"];
    
// Set the action to link annotation.
[linkAnnot setAction:uriAction];
    
// Reset appearance stream.
 [linkAnnot resetAppearanceStream];
    
// Save the document that has added the link annotaiton.
[document saveAs:[DOCUMENT_PATH stringByAppendingString:@"Sample_annot.pdf"] save_flags:FSPDFDocSaveFlagNormal];