making full pdf bold

The reason for me doing this is that on the Kindle text is very small and light colored and hard to read.

By @SLUSHY219625729vve

It sounds like you want accessibility controls in the PDF file so that you can adjust the text and make it more readable for you. There are much better solutions than editing 100+ pages of a textbook!

Are you sure you're reading a PDF file on the Kindle, and not a .MOBI or .EPUB file? Kindles were designed to read .MOBI files, not PDFs, although they can on some models but their adjustments are very limited. Usually PDF files are converted to MOBI to be read on Kindles.

For the best accessibility controls, try to use an Adobe program such as Adobe Acrobat Reader (a free version that reads PDFs) or Adobe Acrobat (the full program that lets you read and create PDFs). Both Adobe and Microsoft are the leading companies that build accessibility features (like adjusting the text) into their software.

Here are some other tactics you can try:

1. On the Kindle, see if it will adjust the font and size for you. Directions are at Amazon's website, https://www.amazon.com/gp/help/customer/display.html?nodeId=GFXBHXY4YXFNFPUE&ref_=hp_help_c_Enlarge-the-Book-Font-on-Your- 
   If your file is a PDF, this may or may not work.
    
2. Download and install a free PDF Reader app on your Kindle. It could give you controls to enlarge the text. See this thread https://community.adobe.com/t5/acrobat-reader-discussions/acrobat-reader-for-kindle-fire/m-p/10520960  As noted, Amazon removed the Kindle's ability to read a PDF. That has nothing to do with Adobe or PDFs in general.
    
3. Read the PDF on any other device BUT a Kindle, and install Adobe Acrobat Reader (free download from https://www.adobe.com/acrobat/pdf-reader.html  Download the free Reader, not the expensive Pro or Standard versions.
   — Try an iPad or other brand of e-reader/tablet with the free Adobe Acrobat Reader.
   — Regular laptop and desktop computer with the free Adobe Acrobat Reader.
   — iPhone or Android with Adobe's Reader for Mobile. https://www.adobe.com/acrobat/mobile/acrobat-reader.html  You just need the free version, the extra features cost $. 
    
4. Look into using a program called Read&Write. It was developed specifically for making book files accessible and readable for those with vision dificiencies.  Easy to use and has a lot more features for reading text-heavy books. Most universities and school districts recommend it for their students: https://academicguides.waldenu.edu/disability-services/assistive-technology/read-write

Hope this helps.

Sure you can:

Click and drag using the "Edit" tool to select all text boxes, then right-click on any blue line : Edit using Illustrator.

Do what you have to do with Illustrator, then Save and Close.

Changes are instantly reported in the PDF file.

Best bet might be to try exporting to Word. If it works, you can mess with the font and size as much as you like, then make a new PDF. Most textbooks will be locked to prevent this though.

No, scripts cannot change the font of page contents. PDF is not made for this. Even if they were, bold fonts are wider than regular fonts and you'd run into nasty reflow issues.

You can't. The purpose of editing in a PDF is for simple things, nothing so major (even if simple as you wish to do).

to do what you want is best done (and intended to be done), in the original creation application. 

sorry,