We received a PDF file with hidden documents so I open the reader but I still could not find it. The first time was easy so it was in attachment. This time I just could not find it anywhere. Can anyone assistance? Thanks in advance for your assistance.
Ask the creater of the file.
If this is some homework (and you, therefore, cannot follow Bernd's advice to ask the author), consider inspecting the internals of the PDF. Adobe Acrobat in its Preflight tool offers the option to Browse Internal PDF Structure; there are other similar tools out there, too, e.g. iText RUPS and PDFBox PDFDebugger. Also you can try and open the PDF in a normal text editor and look for the hidden data, they may be in plain sight there.
What do you mean by "hidden documents"? How do you know they are there in the first place?
Somehow I still did not figure out, but I went back to original documents then I was able to review all the hidden documents. Just share you some update as maybe sometimes the origial document is the way to find the answer. It may not relate to the program or app itself.