Combine PDF files with Prawn

I am having a problem with the combination of several PDF files.

I am generating the PDF using PrawnPDF on the "show" action.

I would like to be able to combine multiple selected items into a single PDF.

I am trying to solve it this way using the combine_pdf geam

def nested_jobs_multiply_pdf  pdf_file_paths  = params[:items]  pdf_file_paths  = pdf_file_paths.map! do |x|    x.to_s +".pdf"  end  @pdfForms = CombinePDF.new  pdf_file_paths.each do |x|    @pdfForms << CombinePDF.parse(nested_job_path(x, format: "pdf"))  end  @pdfForms.save "combined.pdf"  flash[:notice] = "Combined PDF successful"  redirect_to nested_jobs_pathend

Unfortunately, I am getting an error

RuntimeError in NestedJobsController#nested_jobs_multiply_pdfUnknown PDF parsing error - malformed PDF file?Extracted source (around line #89):87   @pdfForms = CombinePDF.new88    pdf_file_paths.each do |x|89    @pdfForms << CombinePDF.parse(nested_job_path(x, format: "pdf"))90   end91  @pdfForms.save "combined.pdf"92