What happens during conversion
A PDF describes text as text: letters, font, position. A JPG knows nothing but pixels. Converting therefore photographs the page, and everything that looks like writing afterwards is really a pattern of dots. Nothing in it can be searched or selected any more.
That is not a flaw, it is often the whole point. An image can be dropped into a presentation, sent in a message or placed on a web page without the recipient having to open a PDF at all.
Resolution decides
With PDF to JPG resolution is the setting that matters most.
72 to 96 dpi is enough for previews and images shown small.
150 dpi is the usable middle ground. Body text stays readable on screen.
300 dpi is what you need if the image gets printed or somebody will zoom in. Files grow accordingly.
If text turns ragged, the resolution was too low. That cannot be repaired afterwards, because the missing pixels cannot be recovered.
JPG or PNG
JPG compresses by throwing away image information. On photographs that goes unnoticed, on sharp edges it does not: faint shadows appear around black letters on white. For pages made mostly of text, tables or drawings, PDF to PNG is the better choice. PNG works without loss, the file gets bigger, the edges stay clean.
The way back
Turning images into a PDF again works with JPG to PDF. The text stays an image though. To get it back you need text recognition.
How many pixels you really need
An A4 page measures 8.27 by 11.69 inches. At 150 dpi that gives an image of roughly 1240 by 1754 pixels, more than enough for any monitor. At 300 dpi it is 2480 by 3508 pixels, four times the data for a difference you cannot see on screen.
The rule of thumb: anything staying on a monitor manages at 150 dpi. Only what gets printed or heavily enlarged needs 300.
Where the detour through an image makes sense
Presentations. A PDF page as an image can be dragged into a slide and placed freely. That beats keeping the PDF alongside and switching during the talk.
Messages and social networks. Many services display images directly and show PDFs only as an attachment nobody opens.
Thumbnails on a web page. An image of the first page as a preview for a download is common and useful.
Documents nobody should edit. No text can be copied out of an image. That is weak protection though, since text recognition gets it back. For real protection there is Protect PDF.
Common questions
Does every page become its own image? Yes, a twenty page PDF yields twenty files. If you only need one page, pull it out first with Extract pages.
Why does the result look paler than the original? Usually the colour profile. Print PDFs often work in CMYK, screen images in RGB. Strong colours therefore shift slightly after conversion.
Can I turn a multipage PDF into one long image? Not directly. The usual route is to produce the individual images and stack them in an image editor afterwards.
What happens to transparent areas? JPG knows no transparency and fills it with white. If you need transparency, use PNG.