在André Lemos的帮助下，我找到了一个解决方案。我将在这里发布完整的实现。
在另一篇文章中，我们必须创建一个实现ITextExtractionStrategy接口的自定义类。也许iText中的抽象类可以省去这一步。

//To be able to extract text from our pdf document
    public class CustomTextExtractionStrategy : ITextExtractionStrategy
    {
        private string textResult { get; set; }

        public CustomTextExtractionStrategy()
        {
        }

        public void EventOccurred(IEventData data, EventType type)
        {
            if (type is EventType.RENDER_TEXT)
            {
                textResult += ((TextRenderInfo)data).GetText();
                
            }
        }

        public string GetResultantText()
        {
            return textResult ?? string.Empty;
        }

        public ICollection<EventType> GetSupportedEvents()
        {
            return new List<EventType>{
                EventType.BEGIN_TEXT,
                EventType.RENDER_TEXT,
                EventType.END_TEXT,
                //EventType.RENDER_IMAGE,
                EventType.RENDER_PATH,
                EventType.CLIP_PATH_CHANGED };
        }
    }

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然后，我们可以将此类用作PdfCanvasProcessor中的参数来处理文本，并通过获取CASTED事件来处理

public class ITextBenchmark
{
    public ITextBenchmark()
    {
    }
    // Our text extracting method
    public IEnumerable<string> TextExtraction()
    {
        var pdftext = new StringBuilder();
        CustomTextExtractionStrategy customTextExtractionStrategy = new CustomTextExtractionStrategy();
        PdfDocument pdfDocument = new PdfDocument(new PdfReader(RessourcesBenchmark.PDF_MERGE_PAGE_TO_APPEND_PATH));
        PdfCanvasProcessor parser = new PdfCanvasProcessor(customTextExtractionStrategy);

        //Process all the pages of the document
        for (int i = 1; i <= pdfDocument.GetNumberOfPages(); i++)
        {
            parser.ProcessPageContent(pdfDocument.GetPage(i));

            // Here we have to extract data from the event listener contained in the parser. 
            var eventResult = (CustomTextExtractionStrategy)parser.GetEventListener();
            pdftext.Append(eventResult.GetResultantText());
            parser.Reset();
        }

        if (pdftext.ToString().Contains("*"))
        {
            return new List<string> { $"The pdf contains '*'" };
        }
        else
        {
            return new List<string> { "Unavailable" };
        }
    }
}

型

有一种方法可以使用C# * 从PDF中检索文本，而不需要 * 使用自定义文本提取策略。使用NET Core很简单，只需要itext7 nuget包。（可以在here中找到工作解决方案的链接）。

using iText.Kernel.Pdf;
using iText.Kernel.Pdf.Canvas.Parser;
using iText.Kernel.Pdf.Canvas.Parser.Listener;

namespace ScanTextInPDFs
{
    internal class Program
    {
        public static async Task Main(string[] args)
        {
            string executingDirectory = AppContext.BaseDirectory;

            byte[] bytes = await File.ReadAllBytesAsync($"{executingDirectory}PDFs\\Brochure.pdf");
            string textToFind = "Lorem ipsum";
            bool foundText = false;

            using (MemoryStream memoryStream = new MemoryStream(bytes))
            {
                using PdfReader pdfReader = new PdfReader(memoryStream);
                using PdfDocument pdfDocument = new PdfDocument(pdfReader);

                for (int page = 1; page <= pdfDocument.GetNumberOfPages(); page++)
                {
                    PdfPage pdfPage = pdfDocument.GetPage(page);
                    string pageText = PdfTextExtractor.GetTextFromPage(pdfPage, new SimpleTextExtractionStrategy());

                    if (pageText.Contains(textToFind, StringComparison.Ordinal))
                        foundText = true;
                }
            }

            if (foundText)
                Console.WriteLine($"Found '{textToFind}' in the pdf.");
            else
                Console.WriteLine($"Did not find '{textToFind}' in the pdf.");
        }
    }
}

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