Question

I’ve tried PDFSharp but they are having issues with the latest versions of Acrobat Reader.

Does anyone know the best ones out there that are free?

I’m coding in C#.

Answer 1

Docotic.Pdf library easily merges PDF files.

Here is a sample that shows how to merge a number of PDFs into one.

using (PdfDocument pdf = new PdfDocument())
{
    string[] filesToMerge = ...
    foreach (string file in filesToMerge)
        pdf.Append(file);

    // delete the first (automatically inserted) page
    pdf.RemovePage(0);

    // document gets saved compressed by default
    // you can compress it even more if you set following options to true

    //pdf.SaveOptions.RemoveUnusedObjects = true;
    //pdf.SaveOptions.UseObjectStreams = true;

    pdf.Save("merged.pdf");
}

Disclaimer: I work for the vendor of the library.