As Gerry said, the control you're using doesn't support loading files from a stream. You have to save them to disk first.
byte[] ap = (byte[])documentsDataGridView.SelectedRows[0].Cells["pdf_file"].Value;
string tempName = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName() + ".pdf");
File.WriteAllBytes(tempName, ap);
axAcroPDF1.src = tempName;
NB: You'll need some code to delete the temporary file when you've finished with it, or when your application closes.
If you want a control that supports loading directly from the stream, try
PdfiumViewer[
^], which uses
Google's Pdfium library[
^]. If you install it via NuGet, pay attention to
the installation instructions[
^] - you'll need one or more additional packages to install the native Pdfium library.