"Just change the date on page two" sounds like a thirty-second job. With a Word document it is. With a PDF it can take thirty seconds or an hour, depending on how the file was made and what software you have. Knowing why saves a lot of frustration, and points straight to the method that will work for your file.
Why PDF text is hard to edit
A Word document stores paragraphs: text that flows, wraps and reflows when you type. A PDF stores the finished page: each line, sometimes each word or letter, placed at an exact position. There is no paragraph to reflow. Change a word to a longer one and the PDF has no idea that the rest of the line should move along.
Fonts add a second problem. To keep files small, PDFs usually include only the letters actually used in the document. If the original never used a capital Q, the file does not contain one, and an editor has to substitute a different font that will not quite match.
And a scanned PDF has no text at all, only a picture of text, so there is nothing to edit until the text has been recognised with OCR.
Best option: go back to the original
If the PDF was made from a Word, Excel or Google Docs file, edit that file and export a new PDF. It is the only method that gives a perfect result, and it is worth asking whoever sent you the PDF for the original before trying anything else.
Small changes: free tools
Microsoft Word can open a PDF directly (File, Open, choose the PDF). It converts the PDF into an editable document. Simple, mostly-text documents come through well; complex layouts with columns and tables shift. Edit, then save as PDF again.
LibreOffice Draw, which is free, opens a PDF with each line of text as an editable box. It is good for changing a date, a name or a figure in place, because nothing else on the page moves. It is poor for rewriting paragraphs.
Google Docs can open a PDF from Google Drive (right-click, Open with, Google Docs). It recognises text even in scans, but loses most formatting. Good for rescuing the words, not the layout.
Paid editors
Adobe Acrobat Pro and similar editors can edit text in place and handle reflowing within a text block. If you edit PDFs every week, they are worth it. For an occasional fix, the free routes above are usually enough.
Adding text is easier than changing it
Many "editing" jobs are really additions: filling in blanks on a form, adding a reference number, writing a note. That does not require editing existing text at all. Fill PDF form lets you click anywhere on a page and type, and Sign PDF adds a signature and date.
The cover-and-retype trick, and its danger
A common workaround is to draw a white box over the old text and type the new text on top. On screen it looks edited. But the old text is still in the file underneath the box. Anyone can select it, search for it or copy it out.
For a typo in a newsletter that does not matter. For changing a figure in an invoice or a date on a contract, it is worse than useless: the original is still readable to anyone who looks, and the document now looks tampered with. If information must be removed, remove it properly with Redact PDF, then add the new text.
If you only need the words
Sometimes the goal is not to change the PDF at all, but to reuse its text in a new document. PDF to text pulls out the text so you can paste it into Word or an email and start fresh. It works on any PDF with selectable text.