While the Doc urged Marty to destroy the almanac, he allowed him to keep the hover board.

Out-of-universe, I'm guessing this is some kind of convenient workaround to the plot's development.
But in-universe, why was the almanac burned and the hover board not burned, destroyed, or kept in the future?

The Doc said he didn't invent the time machine to make money, but at the same time he warned of the perils of taking/knowing things outside one's time, so if the almanac could create a paradox and be a danger, the toy could too.

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    It's easy to see that the two items, if brought back to the present, would affect the future to wildly different degrees. – Lt. Commander. Data Jun 19 '20 at 3:22
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    I would upvote this question twice if I could. The hoverboard could have far more impact on future technological development than an almanac. And I never bought that idea of Biff Tannen getting wealthy off the almanac. With the Butterfly Effect and all, after a few weeks, weather would turn out differently and so events leading up to sports competitions would be different. – RichS Jun 19 '20 at 7:54
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    Maybe the hover board contains some future tech that would be impossible to reverse engineer in the 1980s, which would limit how much its presence in the past would change things. – Klaus Æ. Mogensen Jun 19 '20 at 7:54
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    Maybe Doc thought the batteries would catch on fire before anyone had a chance to take it apart. – DavidW Jun 19 '20 at 10:50
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    @RichS several times in the movies, we see future items change to accommodate changes in the timeline. So, why wouldn't the almanac change too, as sporting events changed, teams got different scores, different schedules, etc? – Remy Lebeau Jun 20 '20 at 2:09

Doc isn't highly strict about the future knowledge thing - he brings Marty to the future in the first place, lets him know how his future son and daughter will turn out, whereabouts he lives, how many sequels of Jaws there will be, and lets him do a bit of sight-seeing in Hill Valley, even though "nobody should know too much about their own future". But these are all relatively minor details.

The Almanac, OTOH, is a major record of the future, and could cause serious damage to the timeline (as it ended up doing, albeit in the hands of Biff rather than Marty). This is more along the lines of the car accident, seeing the inside of his house (only a lot bigger) etc. which Doc does try to avoid.

The hoverboard is probably closer to the first instance - it's a gimmick that doesn't really provide any useful information about the future - of course someone could try to reverse engineer it but he probably thinks it's pretty low risk. The risks with the Almanac though are major and obvious.

  • I partially accept the idea, that at first glance the almanac is more of a threat, but it is supposed to be destroyed to prevent changing the events of a first timeline, but the hoverboard its changing the time line just in the same way that does the almanac with Beef, (like I say in the comment above) so at the end, it is as much threat (at least) than the almanac. – riccs_0x Jun 19 '20 at 15:22
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    How so? Biff used the almanac to transform Hill Valley into a corrupt run-down hell while Marty used the hover board to skate... I'd say there's a huge difference! – colmde Jun 27 '20 at 17:46
  • It does not have to be a great thing like the Biff vice city change, there is danger in some as simple as meeting oneself in the future or past, as noted by the Doc, to avoid Marty's girlfriend know things about she, the paradox warning. So if something as simple to view herself or know one thing or two is avoided (remember the letter warning about the lybians that Doc rip apart and letter assemble), then is more concerning an anacronic object. And the hoverboard wasn't used only for skating it did change the Biff 1955-2 and Clara of 1855. – riccs_0x Jun 27 '20 at 18:02

Personal timeline bias. Doc looks at the possibility of timeline disruption from the perspective of an inhabitant of 1985-1989. The almanac can mess up the timeline that led to "1985 to 1989 Doc". The hoverboard can't. It can only mess up the timeline for people living after 1985 - 1989 - and who cares about them?

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    The hoverboard changed the timeline of Beef 1955 and Clara of 1885 , at least. – riccs_0x Jun 19 '20 at 16:03

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