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Create a set of pages in an MS Word file that are section headings or starters. Then, use MS Word's Table of Contents feature to auto-build the Table of Contents, which are links to each of the section pages. Add this file to your PDF file. Lastly, move the section pages to their appropriate places in the document, so that the Table of Contents created in Word helps readers navigate to the different parts of your PDF file. This second option is best when you need to compose opening remarks for each section of your file.
Compress Your File
Large PDF files, especially if they contain various kinds of data - images, graphs, charts, scanned documents, and so on - can be very large. A page of text can be 100 - 120 kb in a PDF, and many people's default smartphone camera settings can produce images in the 1.5 - 4 mb range. You should take steps to reduce the sizes of these documents or images before incorporating them into a PDF. But even after you've done that and compiled the PDF, Adobe Acrobat Pro has a compression tool that can reduce the final file size still further. Importantly, Acrobat's success in compressing a file depends on its contents; some PDFs may reduce in size dramatically, others less so.
Video Tutorial: Compress a PDF in Acrobat Pro
With the PDF open in Acrobat Pro, on the lefthand side tool menu, choose Compress a PDF. Choose Single File.
In follow-on dialogs, Acrobat asks you to choose a folder, and create a file name. Then click Save, and you should have a compressed copy of your PDF.