Don't modify the document, just print it as an enlargement using the print dialog/print drivers. Everything will get bigger this way, not just the text and there will be no reflow. This may or may not cause the image to span multiple pages, and if your driver can't handle spanning to another page, it may cut off the document.

Slipped in the middle there you could also print the same way, as an enlargement to a PDF if it allows you to span correctly. And then print the resulting pages normally.

To modify PDF files you'll need third party software. I'm not sure what you do with Acrobat other than creating PDFs or concatenating multiple PDFs. Normally this isn't an issue because one would have the original documents for any PDFs they'd created and edits could be done there. You might have better luck printing the PDF from Acrobat than from another program you might be using to view the PDF file.

PDF files can also be opened using Adobe Illustrator which can be an easy way to make changes such as enlargements and substitutions - for people who have Illustrator...