Readiris has a demo version.
Most scanners these days seem to scan direct to PDF.
If the print on the typewritten pages are light, one option would be to take the pages to a self-serve copier and make copies yourself, putting them on the darkest setting so you get nice, dark copies. Then scan the copies.
How old is your scanner? For typewritten pages, even an old scanner with 300dpi resolution should do the job.
One of my relatives wrote a diary which I want to digitize to ensure it's never lost.
What are your expectations regarding "never lost"?
For digital preservation, scan to PDF. You can use OCR/typing too if you want to preserve the text as text rather than a scanned image. Keep both the scanned PDF and the OCR'd/typed text file. That way, you preserve an electronic version of the original document as well as the text.
Keep the digital files on multiple forms of media. At least two. That means hard drive and maybe a CD/DVD. M-DISC if possible too. Keep backups. An off-site backup is also recommended. Also, a physical hard drive, rather than SSD, would be preferable. Store CDs/DVDs in a dry, temperature-controlled, dark place. Keep backup hard drives in a suitable location. Put in a Faraday cage if possible.
For physical preservation, make copies on acid-free, archival-quality paper. Use either a high-end copy machine or digital press or an inkjet printer. Less expensive copy machines or home laser printers may be prone to sticking if the pages are exposed to pressure (stored under/between books) or heat. Keep a copy in a waterproof, fire-proof safe. Keep an off-site copy. Distributing copies to other relatives may also be a good idea.
