To generate PDF file from String Content using Salesforce Apex, use toPdf(). Check the following sample code for reference.

Sample Code:

Blob blobContent = Blob.toPdf( 'Testing File' );
ContentVersion objCV = new ContentVersion(
    Title = 'Test',
    PathOnClient = 'test.pdf',
    VersionData = blobContent,
    IsMajorVersion = true,
    ContentLocation = 'S'
);
insert objCV;
objCV = [
    SELECT ContentDocumentId
    FROM ContentVersion
    WHERE Id =: objCV.Id
];
ContentDocumentLink objCDL = new ContentDocumentLink(
    ContentDocumentId = objCV.ContentDocumentId,
    LinkedEntityId = '0013t00002XXRyDAAX',
    Visibility = 'AllUsers'
);
insert objCDL;

Output: