How to compress PDF file before upload

You can utilize third-party libraries that provide PDF compression functionality. One such library is pdf-lib. Here's an example of how you can compress a PDF file before uploading it in React using pdf-lib

Spire.PDF library is capable of compressing PDF files. 

Refer to the below article:

https://www.e-iceblue.com/Tutorials/Spire.PDF/Spire.PDF-Program-Guide/Document-Operation/How-to-Compress-PDF-Document-in-C-VB.NET.html

One solution is to compress the file via use of a backend service which would allow you to levearge the PDFOptimizierOptions class. This would provide extensive control for controlling the quality and size of the file, and it would return a compressed PDF tailored to your needs. Here is some sample code that demonstrates some options:

using (RasterCodecs _codecs = new RasterCodecs()) 
    { 
        // Set the path to the PDF utilities dll 
        _codecs.Options.Pdf.InitialPath = @"C:\LEADTOOLS22\Bin\Dotnet4\x64"; 
 
        string[] _pdfFiles = Directory.GetFiles(_pdfFileDir, "*.pdf"); 
        string _destDir = Path.Combine(_pdfFileDir, "OptimizedPDFs"); 
 
        // Create the optimizer options 
        PDFOptimizerOptions myOptimizerOptions = new PDFOptimizerOptions(); 
        myOptimizerOptions.AutoOptimizerMode = PDFAutoOptimizerMode.BestSize; 
 
        myOptimizerOptions.ColorImageDownsamplingMode = PDFDownsamplingMode.Average; 
        myOptimizerOptions.GrayImageDownsamplingMode = PDFDownsamplingMode.Bicubic; 
        myOptimizerOptions.MonoImageDownsamplingMode = PDFDownsamplingMode.Bicubic; 
 
        myOptimizerOptions.ColorImageDownsampleFactor = 2.0; 
        myOptimizerOptions.GrayImageDownsampleFactor = 2.0; 
        myOptimizerOptions.MonoImageDownsampleFactor = 2.0; 
 
        myOptimizerOptions.ColorImageDPI = 150; 
        myOptimizerOptions.GrayImageDPI = 150; 
        myOptimizerOptions.MonoImageDPI = 150; 
 
        myOptimizerOptions.ColorImageCompression = RasterImageFormat.Jpeg; 
        myOptimizerOptions.GrayImageCompression = RasterImageFormat.RasPdfLzw; 
        myOptimizerOptions.MonoImageCompression = RasterImageFormat.Jbig; 
 
        myOptimizerOptions.EmbedAllFonts = false; 
        myOptimizerOptions.SubsetFonts = true; 
 
        foreach (string _pdfFile in _pdfFiles) 
        { 
            string _pdfFileName = Path.GetFileNameWithoutExtension(_pdfFile); 
            string _destFile = Path.Combine(_destDir, $"{_pdfFileName}_OPTIMIZED.pdf"); 
 
            PDFFile _pdf = new PDFFile(_pdfFile); 
            _pdf.OptimizerOptions = myOptimizerOptions; 
            _pdf.Optimize(_destFile);
        }

https://www.leadtools.com/help/sdk/tutorials/dotnet-console-optimize-pdf-files-for-storage.html
https://www.leadtools.com/help/sdk/dh/pdf/pdfoptimizeroptions.html